Summary:
Recently, pipelined write is enabled even if atomic flush is enabled, which causing sanitizing failure in db_stress. Revert this change.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5986

Test Plan: Run "make crash_test_with_atomic_flush" and see it to run for some while so that the old sanitizing error (which showed up quickly) doesn't show up.

Differential Revision: D18228278

fbshipit-source-id: 27fdf2f8e3e77068c9725a838b9bef4ab25a2553