Summary:
Atomic flush is incompatible with pipelined write. At least now.
If pipelined write is enabled, a thread performing write can exit the write
thread and start inserting into memtables. Consequently a thread performing
flush will enter write thread and race with memtable insertion by the former.
This will cause undefined result in terms of data persistence.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5860
Test Plan:
```
$make all && make check
```
Differential Revision: D17638944
Pulled By: riversand963
fbshipit-source-id: abc578dc49a5dbe41bc5adcecf448f8e042a6d49
643df920d8