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.
(cherry picked from commit 4e1faa35d5)
21771f5295