Summary:
Sometimes we want to compact files as fast as possible, but don't want to set a large `max_subcompactions` in the `DBOptions` by default.
I add a `max_subcompactions` options to `CompactionOptions` so that we can choose a proper concurrency dynamically.
Closes https://github.com/facebook/rocksdb/pull/3775

Differential Revision: D7792357

Pulled By: ajkr

fbshipit-source-id: 94f54c3784dce69e40a229721a79a97e80cd6a6c