Pause the dbiterator before calling dns_db_find

zone.c:integrity_checks() acquires a read lock while iterating the
zone database, and calls zone_check_mx() which acquires another
read lock. If another thread tries to acquire a write lock in the
meantime, it can deadlock. Calling dns_dbiterator_pause() to release
the first read lock prevents this.

lib/dns/zone.c

@@ -3326,6 +3326,8 @@ integrity_checks(dns_zone_t *zone, dns_db_t *db) {
 			goto next;
 		}
 
+		dns_dbiterator_pause(dbiterator);
+
 		/*
 		 * Don't check the NS records at the origin.
 		 */