Summary:
This PR adds support to the c-api bindings for calling `Flush()` with multiple column families, which is useful for performing atomic flushes (assuming also that the db has been opened with `atomic_flush = true`).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11112

Reviewed By: cbi42

Differential Revision: D42666382

Pulled By: ajkr

fbshipit-source-id: 82f05bf32d28452d85c79ea42411c8fea961fd87