Fix DBFlushTest::ManualFlushWithMinWriteBufferNumberToMerge dead lock

Summary:
In the test, there can be a dead lock between background flush thread and foreground main thread as following:
* background flush thread:
  - holding db mutex, while
  - waiting on "DBImpl::FlushMemTableToOutputFile:BeforeInstallSV" sync point.
* foreground thread:
  - waiting for db mutex to write "key2"

Fixing by let background flush thread wait without holding db mutex.
Closes https://github.com/facebook/rocksdb/pull/3436

Differential Revision: D6841334

Pulled By: yiwu-arbug

fbshipit-source-id: b020768ac94e166e40953c5d09e505515a5f244d

db/db_flush_test.cc

@@ -137,7 +137,7 @@ TEST_F(DBFlushTest, ManualFlushWithMinWriteBufferNumberToMerge) {
       {{"DBImpl::BGWorkFlush",
         "DBFlushTest::ManualFlushWithMinWriteBufferNumberToMerge:1"},
        {"DBFlushTest::ManualFlushWithMinWriteBufferNumberToMerge:2",
-        "DBImpl::FlushMemTableToOutputFile:BeforeInstallSV"}});
+        "FlushJob::WriteLevel0Table"}});
   SyncPoint::GetInstance()->EnableProcessing();
 
   ASSERT_OK(Put("key1", "value1"));

db/db_impl_compaction_flush.cc

@@ -134,7 +134,6 @@ Status DBImpl::FlushMemTableToOutputFile(
   }
 
   if (s.ok()) {
-    TEST_SYNC_POINT("DBImpl::FlushMemTableToOutputFile:BeforeInstallSV");
     InstallSuperVersionAndScheduleWork(cfd, &job_context->superversion_context,
                                        mutable_cf_options);
     if (made_progress) {