Summary:
A race condition will happen when:
* a user thread writes a value, but it hits the write stop condition because there are too many un-flushed memtables, while holding blob_db_impl.write_mutex_.
* Flush is triggered and call flush begin listener and try to acquire blob_db_impl.write_mutex_.
Fixing it.
Closes https://github.com/facebook/rocksdb/pull/3149
Differential Revision: D6279805
Pulled By: yiwu-arbug
fbshipit-source-id: 0e3c58afb78795ebe3360a2c69e05651e3908c40
e58d377182