Summary:
Currently, `DeleteDir` only deletes the directory if there are no other directories under the target dir. This PR makes it delete directories recursively.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6934

Test Plan:
Added a new unit test in testutil_test.cc.
`make testutil_test`

Reviewed By: zhichao-cao

Differential Revision: D21884211

Pulled By: cheng-chang

fbshipit-source-id: 0b9a48a200f494ee007aef5d1763b4aa331f8b5a