...in case when update is re-scheduled by zone_update_enqueue()

This does not big harm on not-frequently updated zones, NOTIFY is simply delayed.
This helps on frequently updated (DDNS or [AI]XFR) zones - NOTIFY is not sent more often than once a second.

Libor Peltan authored 8 years ago and Daniel Salzman committed 8 years ago

instead, if obtaining key by keytag is necessary, we walk through all the matching keys and choose the appropriate one

- We have to cancel the timer before scheduling for a fixed time.
- We cannot cancel the timer if the zone is expired based on time,
  because the expire handler may not have been called yet.