Summary:
Adding some stats that would be helpful to monitor if the DB has gone to unlikely stats that would hurt the performance. These are mostly when we end up needing to acquire a mutex.
Closes https://github.com/facebook/rocksdb/pull/3683

Differential Revision: D7529393

Pulled By: maysamyabandeh

fbshipit-source-id: f7d36279a8f39bd84d8ddbf64b5c97f670c5d6d9