Summary:
We see crash test occassionally fails with "A checkpoint operation failed with: Invalid argument: Directory exists". The suspicious is that the directory fails to be deleted because some trash files. Deep clean the directory after a DestroyDB() call.

Also add more debugging printf in case it fails.
Also, preserve the DB if verification fails.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7039

Test Plan: Run db_stress with low --checkpoint_one_in value

Reviewed By: riversand963

Differential Revision: D22271694

fbshipit-source-id: 6a9b2abb664fc69a4dc666741df4f6b23703cd6d