timer: rate-limit the "timer expiry in the past" error messages

We already ratelimit the normal notification about event processing lagging behind but in the case of timers actually expiring late, we'd pass those messages on. So lots of clicks on a slow-reponse system resulted in lots of messages triggered by the debounce timers.

Use the same ratelimiting as the event processing warning, 5 messages per hour which should be a good balance between warning and not spamming the log.

Fixes #711 (closed)