When the clock jumps, and we have a record of last user activity,
adjust that record. This way if I'm inactive for 10 minutes and
then the laptop is sleeping for an hour, I'll still count as having
been inactive for 10 minutes.
Previously, we treat every jump as if it were activity, which is
ridiculous, and would prevent a Tor instance with a jumpy clock from
ever going dormant.
Rather than initializing the "Dormant" status to "off" and the "last
activity" count to "now", initialize them based on our state file:
stay dormant if we were dormant, or remember the amount of time
we've spent inactive.
Since this is completely core functionality, I'm putting it in
core/mainloop, even though it depends on feature/hibernate. We'll
have to sort that out in the future.
