Summary:
I'm going with brute-force solution, just letting Put() and Write() holding a mutex before writing. May improve concurrent writing with finer granularity locking later.
Closes https://github.com/facebook/rocksdb/pull/2682
Differential Revision: D5552690
Pulled By: yiwu-arbug
fbshipit-source-id: 039abd675b5d274a7af6428198d1733cafecef4c