Summary:
Post-compaction work holds onto db mutex for the longest time (found by tracing lock acquires/releases with LTTng and correlating timestamps with our info log). Further experimentation showed `TableCache::EraseHandle` is responsible for ~86% of time mutex is held. We can just release the handle outside the db mutex.
Closes https://github.com/facebook/rocksdb/pull/2654

Differential Revision: D5507126

Pulled By: ajkr

fbshipit-source-id: 703c01ddf2aea16bc0f9e33c08935d78aa6b781d