Add restore worker

A restore worker was added to keep track of restore status.
It is to be used for the MachineAgent in cmd/jujud to know when to
restrict login and for the final step of restore to communicate
a succesful run.
the part of this error message after the colon seems questionable.  Guessing as to the reason for an error is almost always a bad idea.  Either we know something changed while this happened, or we don't know, and shouldn't guess.