Process db callbacks in zone_loaddone() after zone_postload()

Process db callbacks in zone_loaddone() after zone_postload()

The zone_postload() function can fail and unregister the callbacks.

Call dns_db_endload() only after calling zone_postload() to make
sure that the registered update-notify callbacks are not called
when the zone loading has failed during zone_postload().

Also, don't ignore the return value of zone_postload().

(cherry picked from commit ed268b46f11706bde3da68bd2a4b45752350f736)