Summary:
After unclean crash, the tail of the log could look as follows due to block buffering, even when the call to `ROCKSDB_LOG_ERROR()` finished.
```
2020/09/29-13:54:39.596710 7f67025fe700 [ERROR] [/db_impl/db_impl_compaction_flush.cc:2500] Waiting after background compaction err
```
This PR forces the flush while logging warning severity or higher to prevent that case.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7462
Reviewed By: riversand963
Differential Revision: D24000154
Pulled By: ajkr
fbshipit-source-id: 3bf5f1e69a62ee10e84095cebc88937a8f81b4ad
1600aac46f