Bump up memory order of ref counting of ColumnFamilyData (#5723)

Summary:
We see this TSAN warning:

WARNING: ThreadSanitizer: data race (pid=282806)
  Write of size 8 at 0x7b6c00000e38 by thread T16 (mutexes: write M1023578822185846136):
    #0 operator delete(void*) <null> (libtsan.so.0+0x0000000795f8)
    https://github.com/facebook/rocksdb/issues/1 rocksdb::DBImpl::BackgroundFlush(bool*, rocksdb::JobContext*, rocksdb::LogBuffer*, rocksdb::FlushReason*, rocksdb::Env::Priority) db/db_impl/db_impl_compaction_flush.cc:2202 (db_flush_test+0x00000060b462)
    https://github.com/facebook/rocksdb/issues/2 rocksdb::DBImpl::BackgroundCallFlush(rocksdb::Env::Priority) db/db_impl/db_impl_compaction_flush.cc:2226 (db_flush_test+0x00000060cbd8)
    https://github.com/facebook/rocksdb/issues/3 rocksdb::DBImpl::BGWorkFlush(void*) db/db_impl/db_impl_compaction_flush.cc:2073 (db_flush_test+0x00000060d5ac)
    ......

Previous atomic write of size 4 at 0x7b6c00000e38 by main thread:
    #0 __tsan_atomic32_fetch_sub <null> (libtsan.so.0+0x00000006d721)
    https://github.com/facebook/rocksdb/issues/1 std::__atomic_base<int>::fetch_sub(int, std::memory_order) /mnt/gvfs/third-party2/libgcc/c67031f0f739ac61575a061518d6ef5038f99f90/7.x/platform007/5620abc/include/c++/7.3.0/bits/atomic_base.h:524 (db_flush_test+0x0000005f9e38)
    https://github.com/facebook/rocksdb/issues/2 rocksdb::ColumnFamilyData::Unref() db/column_family.h:286 (db_flush_test+0x0000005f9e38)
    https://github.com/facebook/rocksdb/issues/3 rocksdb::DBImpl::FlushMemTable(rocksdb::ColumnFamilyData*, rocksdb::FlushOptions const&, rocksdb::FlushReason, bool) db/db_impl/db_impl_compaction_flush.cc:1624 (db_flush_test+0x0000005f9e38)
    https://github.com/facebook/rocksdb/issues/4 rocksdb::DBImpl::TEST_FlushMemTable(rocksdb::ColumnFamilyData*, rocksdb::FlushOptions const&) db/db_impl/db_impl_debug.cc:127 (db_flush_test+0x00000061ace9)
    https://github.com/facebook/rocksdb/issues/5 rocksdb::DBFlushTest_CFDropRaceWithWaitForFlushMemTables_Test::TestBody() db/db_flush_test.cc:320 (db_flush_test+0x0000004b44e5)
    https://github.com/facebook/rocksdb/issues/6 void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) third-party/gtest-1.7.0/fused-src/gtest/gtest-all.cc:3824 (db_flush_test+0x000000be2988)
    ......

It's still very clear the cause of the warning is because that TSAN treats results from relaxed atomic::fetch_sub() as non-atomic with the operation itself. We can make it more explicit by bumping up the order to CS.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5723

Test Plan: Run all existing test.

Differential Revision: D16908250

fbshipit-source-id: bf17d39ed19058372bdf97f6440a743f88153021

db/column_family.h

@@ -275,7 +275,7 @@ class ColumnFamilyData {
   // Ref() can only be called from a context where the caller can guarantee
   // that ColumnFamilyData is alive (while holding a non-zero ref already,
   // holding a DB mutex, or as the leader in a write batch group).
-  void Ref() { refs_.fetch_add(1, std::memory_order_relaxed); }
+  void Ref() { refs_.fetch_add(1); }
 
   // Unref decreases the reference count, but does not handle deletion
   // when the count goes to 0.  If this method returns true then the
@@ -283,7 +283,7 @@ class ColumnFamilyData {
   // FreeDeadColumnFamilies().  Unref() can only be called while holding
   // a DB mutex, or during single-threaded recovery.
   bool Unref() {
-    int old_refs = refs_.fetch_sub(1, std::memory_order_relaxed);
+    int old_refs = refs_.fetch_sub(1);
     assert(old_refs > 0);
     return old_refs == 1;
   }