The dns_rpz_zones structure was using .refs and .irefs for strong and
weak reference counting. Rewrite the unit to use just a single
reference counting + shutdown sequence (dns_rpz_destroy_rpzs) that must
be called by the creator of the dns_rpz_zones_t object. Remove the
reference counting from the dns_rpz_zone structure as it is not needed
because the zone objects are fully embedded into the dns_rpz_zones
structure and dns_rpz_zones_t object must never be destroyed before all
dns_rpz_zone_t objects.
The dns_rps_zones_t reference counting uses the new ISC_REFCOUNT_TRACE
capability - enable by defining DNS_RPZ_TRACE in the dns/rpz.h header.
Additionally, add magic numbers to the dns_rpz_zone and dns_rpz_zones
structures.
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