Summary:
Adds a DB option `atomic_flush` to control whether to enable this feature. This PR is a subset of [PR 3752](https://github.com/facebook/rocksdb/pull/3752).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4023

Differential Revision: D8518381

Pulled By: riversand963

fbshipit-source-id: 1e3bb33e99bb102876a31b378d93b0138ff6634f