Summary:
If crash happen after a hard link established, Recover function may reuse the file number that has already assigned to the internal file, and this will overwrite the external file. To protect the external file, we have to make sure the file number will never being reused.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4099
Differential Revision: D9034092
Pulled By: riversand963
fbshipit-source-id: 3f1a737440b86aa2ef01673e5013aacbb7c33e28
f5e46354d2