Summary:
Currently log_writer->AddRecord in WriteImpl is protected from concurrent calls via FlushWAL only if two_write_queues_ option is set. The patch fixes the problem by i) skip log_writer->AddRecord in FlushWAL if manual_wal_flush is not set, ii) protects log_writer->AddRecord in WriteImpl via log_write_mutex_ if manual_wal_flush_ is set but two_write_queues_ is not.

Fixes #3599
Closes https://github.com/facebook/rocksdb/pull/3656

Differential Revision: D7405608

Pulled By: maysamyabandeh

fbshipit-source-id: d6cc265051c77ae49c7c6df4f427350baaf46934