[FileBackend]

[FileBackend]
* Added FileJournal class to log file changes for file backends. This can be used for migrations (like moving to Swift), syncing mirror repos, consistency checks, finishing/reverting operation batches, and such. The default journal is the "null" journal, which simply does nothing.
* Added the optional schema change required for using the DBFileJournal (MySQL, SQLite).

Change-Id: I33c9f9a598ba1f164c862b9dc3c718f9172db02b