Summary:
Fix a TSAN failure in `DBRangeDelTest.ValidLevelSubcompactionBoundaries`:
https://gist.github.com/miasantreble/712e04b4de2ff7f193c98b1acf07e899
Closes https://github.com/facebook/rocksdb/pull/3691

Differential Revision: D7541400

Pulled By: miasantreble

fbshipit-source-id: b0b4538980bce7febd0385e61d6e046580bcaefb