Summary:
As the last step in backup creation, the .tmp directory is renamed omitting the .tmp suffix. In case the process terminates before this, the .tmp directory will be left behind. Even if this happens, we want future backups to succeed, so I added some checks/cleanup for this case.
Closes https://github.com/facebook/rocksdb/pull/1896

Differential Revision: D4597323

Pulled By: ajkr

fbshipit-source-id: 48900d8