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.
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