BackupEngine gluster-friendly file naming convention

BackupEngine gluster-friendly file naming convention

Summary:
Use the rsync tempfile naming convention in our `BackupEngine`. The temp file follows the format, `.<filename>.<suffix>`, which is later renamed to `<filename>`. We fix `tmp` as the `<suffix>` as we don't need to use random bytes for now. The benefit is gluster treats this tempfile naming convention specially and applies hashing only to `<filename>`, so the file won't need to be linked or moved when it's renamed. Our gluster team suggested this will make things operationally easier.
Closes https://github.com/facebook/rocksdb/pull/3463

Differential Revision: D6893333

Pulled By: ajkr

fbshipit-source-id: fd7622978f4b2487fce33cde40dd3124f16bcaa8