scheduling - keep zone serial when zone event is created
If multiple events which work with the same zone are present in the scheduling queue (and possibly spawning new events above the same zone) and cause zone content invalidation (KNOT_ZONE_DISCARDED
flag), then it is difficult to cancel all future no longer valid events.
This could be solved by having some increasing counter in the zone structure instead of reference count (zone serial could serve as well). Then when an event above a zone is scheduled, the event should also keep the counter value valid at the moment, when the event was created. Therefore we could easily identify outdated events.