Summary:
Allow users to configure the trash-to-DB size ratio limit, so
that ratelimits for deletes can be enforced even when larger portions of
the database are being deleted.
Closes https://github.com/facebook/rocksdb/pull/3158

Differential Revision: D6304897

Pulled By: gdavidsson

fbshipit-source-id: a28dd13059ebab7d4171b953ed91ce383a84d6b3