Revamp backup scheduling policy

We now schedule a periodic check of pending backups; if any apps have requested
a backup since the last check, we perform all of the pending backups.  The
periodic backup scheduling matches the enable/disable state of the backup
manager; while backups are disabled entirely there are no periodic wakeups.

The period is set here to one hour.  If an external caller (transport, the
'bmgr' command line tool, etc) requests an immediate backup pass, that is
performed and then the periodic backup check is rescheduled using that pass as
the starting point of a new interval.