Don't process messages that are obviously stale

My installation seems to have lost whatever flag it had that allowed it
to skip the backlog when connecting via XMPP, resulting in a slew of
false positives every ~24 hours as dynos restart.

This should prevent most of that by skipping messages that are
obviously stale.
Merge pull request #11 from brandur/brandur-skip-stale

Don't process messages that are obviously stale