Added a backup manifest key, which is used if the main manifest key is
not present. When uploading a new Manifest, it makes sure that it never
drops one key except when the other key is present.
It's entirely possible for the two manifest keys to get out of sync, due
to races. The main one wins when it's present, it is possible for the
main one being dropped to expose the backup one, which has a different
push recorded.
