Summary:
Make `max-subcompactions` dynamically changeable by passing the `DBOption` to Compaction.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7159

Reviewed By: siying

Differential Revision: D22671238

Pulled By: jay-zhuang

fbshipit-source-id: 311ca9f6bb606965544d8708616d358cfed5be42