make flush_reason_ atomic to keep TSAN happy

Summary: Closes https://github.com/facebook/rocksdb/pull/3487

Differential Revision: D6967098

Pulled By: miasantreble

fbshipit-source-id: 48e0accf2e3b3f589ddb797ff8083c8520269bf0

db/column_family.h

@@ -408,7 +408,7 @@ class ColumnFamilyData {
   // recovered from
   uint64_t log_number_;
 
-  FlushReason flush_reason_;
+  std::atomic<FlushReason> flush_reason_;
 
   // An object that keeps all the compaction stats
   // and picks the next compaction