Summary:
MyRocks calls `GetForUpdate` on `INSERT`, for unique key check, and in almost all cases GetForUpdate returns empty result. For such cases, whole key bloom filter is helpful.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4985

Differential Revision: D14118257

Pulled By: miasantreble

fbshipit-source-id: d35cb7109c62fd5ad541a26968e3a3e16d3e85ea