Summary:
The zeroed entries were not removed from prepared_section_completed_ map. This patch adds a unit test to show the problem and fixes that by refactoring the code. The new code is more efficient since i) it uses two separate mutex to avoid contention between commit and prepare threads, ii) it uses a sorted vector for maintaining uniq log entires with prepare which avoids a very large heap with many duplicate entries.
Closes https://github.com/facebook/rocksdb/pull/3545

Differential Revision: D7106071

Pulled By: maysamyabandeh

fbshipit-source-id: b3ae17cb6cd37ef10b6b35e0086c15c758768a48