Summary:
The whole point of the pimpl idiom is to hide implementation details.
Internal helper methods like `CheckConsistency`, `CheckConsistencyForDeletes`,
and `MaybeAddFile` do not belong in the public interface of the class.
In addition, the patch switches to `unique_ptr` for the implementation
object instead of using a raw `delete`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6556

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D20523568

Pulled By: ltamasi

fbshipit-source-id: 5bbb0ccebd0c47a33b815398c7f9cfe13bd775ac