Summary:
If one column family is dropped, we should simply skip it and continue to flush
other active ones.
Currently we use Status::ShutdownInProgress to notify caller of column families
being dropped. In the future, we should consider using a different Status code.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4708

Differential Revision: D13378954

Pulled By: riversand963

fbshipit-source-id: 42f248cdf2d32d4c0f677cd39012694b8f1328ca