More restore plumbing, plus add suggested-backoff to transport API

Adds most of the code for a background-thread restore process, structured much
like the backup thread.  Broke some common functionality out into a helper
function for doing a synchronous wait for a requested agent to attach.

Added a method to IBackupTransport whereby the transport will be asked for
an opinion on whether this is a good time for a backup to happen.  It will
reply with the results of its policymaking around backoff intervals, time-of-day
selection, etc.