[FileRepo]

[FileRepo]
* Added getReadOnlyReason() function to FileRepo to check read-only status. Added such checks to the File object functions that mutate files in storage. This should make read-only mode more tolerable (which is needed at least briefly when switching backends).
* Added lock()/unlock() calls to File restore() function.
* Use proper isOK() accessor for Status objects.

Change-Id: I7368912b01f7fd1ec34125d12c0844388cbb3b2d