Make TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAcces less flaky (#9281)

Summary:
Context:
[Rapid thread creation and deletion](https://github.com/facebook/rocksdb/blob/6.27.fb/utilities/transactions/write_prepared_transaction_test.cc#L439-L444) in  `SnapshotConcurrentAccessTest.SnapshotConcurrentAcces` inside a [potentially big loop](https://github.com/facebook/rocksdb/blob/6.27.fb/utilities/transactions/write_prepared_transaction_test.cc#L1238-L1248) can lead to heavy-loading the system with many threads due to delay in actually cleaning up thread's resource in the kernel sometime. We ran into some [flaky failure](https://app.circleci.com/pipelines/github/facebook/rocksdb/10383/workflows/136f1005-80a9-4515-aee9-fe36ac6462a1/jobs/253289) in CI and reproduced it by below:

- Command
```
Added `ROCKSDB_NAMESPACE::port::InstallStackTraceHandler();` like https://github.com/facebook/rocksdb/pull/9276
DEBUG_LEVEL=2 make -j56 write_prepared_transaction_test
GTEST_CATCH_EXCEPTIONS=0 ~/gtest-parallel/gtest-parallel -r 200 -w 200 ./write_prepared_transaction_test --gtest_filter=TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
```
- Stack, where `write_prepared_transaction_test.cc:442` in `https://github.com/facebook/rocksdb/issues/9` points to thread creation
```
[ RUN      ] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
....terminate called after throwing an instance of 'std::system_error'
  what():  Resource temporarily unavailable
Received signal 6 (Aborted)
#0   /lib/x86_64-linux-gnu/libc.so.6(gsignal+0x38) [0x7fc114f39438]
...
https://github.com/facebook/rocksdb/issues/7   /usr/lib/x86_64-linux-gnu/libstdc++.so.6(+0xb8e73) [0x7fc1158a5e73] ??	??:0
https://github.com/facebook/rocksdb/issues/8   ./write_prepared_transaction_test() [0x4ca86c] std:🧵:thread<rocksdb::WritePreparedTransactionTestBase::SnapshotConcurrentAccessTestInternal(rocksdb::WritePreparedTxnDB*, std::vector<unsigned long, std::allocator<unsigned long> > const&, std::vector<unsigned long, std::allocator<unsigned long> 	 const&, rocksdb::WritePreparedTxnDB::CommitEntry&, unsigned long&, unsigned long, unsigned long, unsigned long, unsigned long)::{lambda()https://github.com/facebook/rocksdb/issues/1}>(rocksdb::WritePreparedTransactionTestBase::SnapshotConcurrentAccessTestInternal(rocksdb::WritePreparedTxnDB*, s	d::vector<unsigned long, std::allocator<unsigned long> > const&, std::vector<unsigned long, std::allocator<unsigned long> > const&, rocksdb::WritePreparedTxnDB::CommitEntry&, unsigned long&, unsigned long, unsigned long, unsigned long, unsigned long)::{l	mbda()https://github.com/facebook/rocksdb/issues/1}&&)	/usr/include/c++/5/thread:137 (discriminator 4)
https://github.com/facebook/rocksdb/issues/9   ./write_prepared_transaction_test() [0x4bb80c] rocksdb::WritePreparedTransactionTestBase::SnapshotConcurrentAccessTestInternal(rocksdb::WritePreparedTxnDB*, std::vector<unsigned long, std::allocator<unsigned long> > const&, std::vector<unsigned long, std::allocator<unsigned long> > const&, rocksdb::W	itePreparedTxnDB::CommitEntry&, unsigned long&, unsigned long, unsigned long, unsigned long, unsigned long)	/home/circleci/project/utilities/transactions/write_prepared_transaction_test.cc:442
https://github.com/facebook/rocksdb/issues/10  ./write_prepared_transaction_test() [0x4407b6] rocksdb::SnapshotConcurrentAccessTest_SnapshotConcurrentAccess_Test::TestBody()	/home/circleci/project/utilities/transactions/write_prepared_transaction_test.cc:1244
...
[109/200] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1 returned/aborted with exit code -6 (34462 ms)
```

- Move thread 2's work into current thread to avoid half of the thread creation cuz there is no difference in doing so. We expect this can make the thread-creation error less often, even though we can't gurantee it from happening again. Considering this is a trivial change with positive impact, it's still worth landing and monitor if it's enough to solve the problem in reality.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9281

Test Plan:
Before the change, repeating the test 200 times with 200 workers failed
`~/gtest-parallel/gtest-parallel -r 200 -w 200 ./write_prepared_transaction_test --gtest_filter=TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1`

```
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from TwoWriteQueues/SnapshotConcurrentAccessTest
[ RUN      ] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
..unknown file: Failure
C++ exception with description "Resource temporarily unavailable" thrown in the test body.
[  FAILED  ] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1, where GetParam() = (false, true, 1, 0, 1, 20) (11882 ms)
[----------] 1 test from TwoWriteQueues/SnapshotConcurrentAccessTest (11882 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (11882 ms total)
[  PASSED  ] 0 tests.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1, where GetParam() = (false, true, 1, 0, 1, 20)
```

After the change: repeating the test 200 times with 200 workers didn't fail, even with repeating the "repeating" for 10 times like below
`for i in {1..10}; do ~/gtest-parallel/gtest-parallel -r 200 -w 200 ./write_prepared_transaction_test --gtest_filter=TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1; done`

```
[200/200] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
[200/200] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
[200/200] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
[200/200] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
[200/200] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
[200/200] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
[200/200] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
[200/200] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
[200/200] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
[200/200] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
```

It does failed when repeating the test 400 times with 400 workers
`~/project$ ~/gtest-parallel/gtest-parallel -r 400 -w 400 ./write_prepared_transaction_test --gtest_filter=TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1`

```
[1/400] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1 (2928 ms)
Note: Google Test filter = TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from TwoWriteQueues/SnapshotConcurrentAccessTest
[ RUN      ] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1
unknown file: Failure
C++ exception with description "std::bad_alloc" thrown in the test body.
[  FAILED  ] TwoWriteQueues/SnapshotConcurrentAccessTest.SnapshotConcurrentAccess/1, where GetParam() = (false, true, 1, 0, 1, 20) (2597 ms)
[----------] 1 test from TwoWriteQueues/SnapshotConcurrentAccessTest (2597 ms total)
```

Reviewed By: ajkr

Differential Revision: D33026776

Pulled By: hx235

fbshipit-source-id: 509f57126392821e835e48396e5bf224f4f5dcac

utilities/transactions/write_prepared_transaction_test.cc

@@ -438,10 +438,8 @@ class WritePreparedTransactionTestBase : public TransactionTestBase {
       ASSERT_TRUE(wp_db->old_commit_map_empty_);
       ROCKSDB_NAMESPACE::port::Thread t1(
           [&]() { wp_db->UpdateSnapshots(new_snapshots, version); });
-      ROCKSDB_NAMESPACE::port::Thread t2(
-          [&]() { wp_db->CheckAgainstSnapshots(entry); });
+      wp_db->CheckAgainstSnapshots(entry);
       t1.join();
-      t2.join();
       ASSERT_FALSE(wp_db->old_commit_map_empty_);
     }
     ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->DisableProcessing();
@@ -467,10 +465,8 @@ class WritePreparedTransactionTestBase : public TransactionTestBase {
       ASSERT_TRUE(wp_db->old_commit_map_empty_);
       ROCKSDB_NAMESPACE::port::Thread t1(
           [&]() { wp_db->UpdateSnapshots(new_snapshots, version); });
-      ROCKSDB_NAMESPACE::port::Thread t2(
-          [&]() { wp_db->CheckAgainstSnapshots(entry); });
+      wp_db->CheckAgainstSnapshots(entry);
       t1.join();
-      t2.join();
       ASSERT_FALSE(wp_db->old_commit_map_empty_);
     }
     ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->DisableProcessing();