Keep track of backup state independently for each transport

Backup transports now provide the Backup Manager with a suggested name with
which it can disambiguate any transport-specific bookkeeping that it needs to
maintain.  The Manager keeps separate application backup 'state blobs' for each
transport now, preventing things from getting out of step if the device is
switched among multiple transports.

Also, the metadata backup agent is always invoked now on each backup pass.  This
is cheap when there is nothing to do, but also strongly ensures that we never
wind up in a situation where a given transport destination has not been given
all of the metadata necessary for the backup set.