Summary:
This diff introduces some features that were requested by two internal customers:
* Ability for backups not to share table files, because we can't guarantee that equal filename means equal content accross replicas
* Ability for two threads to call EnableFileDeletions() and DisableFileDeletions()
* Ability to stop backup from another thread and not slow down the DB close
* Copy the files to the temporary folder first and then atomically rename

Test Plan: Added some tests to backupable_db_test

Reviewers: dhruba, sanketh, muthu, sdong, haobo

Reviewed By: haobo

CC: leveldb, sanketh, muthu

Differential Revision: https://reviews.facebook.net/D14769