duplicate update_listener registration for rpz db on reload
From Support ticket https://support.isc.org/Ticket/Display.html?id=13211
I've just noticed some awkward behavior when reloading an rpz. It involves calls t dns_zone_rpz_enable_db() both in zone_startload() and zone_replacedb(), so 'db' will have a duplicate 'update_listener' entries. I've not tested it myself but the same thing can happen for inbound AXFR. Is that intentional? And if so, what's the point of it? As far as I can see it doesn't cause a big trouble like server crash or memory leak, but it looked to me like a mere waste and odd. If that's not intentional, you might want to clean it up eventually. If it's intentional, some code comment to explain the intent would help other code readers.