NOTIFY delayed 1s after DDNS or XFR.

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.

src/knot/events/handlers/refresh.c

@@ -918,7 +918,7 @@ int event_refresh(conf_t *conf, zone_t *zone)
 	/* Rechedule events. */
 	replan_from_timers(conf, zone);
 	if (updated) {
-		zone_events_schedule_now(zone, ZONE_EVENT_NOTIFY);
+		zone_events_schedule_at(zone, ZONE_EVENT_NOTIFY, time(NULL) + 1);
 
 		conf_val_t val = conf_zone_get(conf, C_ZONEFILE_SYNC, zone->name);
 		int64_t sync_timeout = conf_int(&val);

src/knot/nameserver/update.c

@@ -200,7 +200,7 @@ static void process_requests(conf_t *conf, zone_t *zone, list_t *requests)
 	              "%.02f seconds", old_serial, new_serial,
 	              time_diff_ms(&t_start, &t_end));
 
-	zone_events_schedule_now(zone, ZONE_EVENT_NOTIFY);
+	zone_events_schedule_at(zone, ZONE_EVENT_NOTIFY, time(NULL) + 1);
 }
 
 static int remote_forward(conf_t *conf, struct knot_request *request, conf_remote_t *remote)