When MPD quits in a non-clean way, the state file isn't written, and
on the next start, MPD time warps to the previous clean shutdown.
Save the state file every 5 minutes; this will probably be
configurable at a later time.

Note that we don't set a wakeup timer for that: when there is no MPD
traffic, MPD won't wake up to save the state file.  This minor bug is
tolerated, because usually there is no change in MPD's state when the
main thread is idle.