Summary:
As title. All core db implementations should stay in db_impl.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8082

Test Plan: make check

Reviewed By: ajkr

Differential Revision: D27211442

Pulled By: riversand963

fbshipit-source-id: e0953fde75064740e899aaff7989ff033b7f5232