Add isc_refcount_destroy() for dns_zt reference counters

The zt_destroy() function was missing isc_refcount_destroy() on the two
reference counters.  The isc_refcount_destroy() adds proper memory
ordering on destroy and also ensures that the reference counters have
been zeroed before destroying the object.

lib/dns/zt.c

@@ -222,6 +222,9 @@ flush(dns_zone_t *zone, void *uap) {
 
 static void
 zt_destroy(dns_zt_t *zt) {
+	isc_refcount_destroy(&zt->references);
+	isc_refcount_destroy(&zt->loads_pending);
+
 	if (atomic_load_acquire(&zt->flush)) {
 		(void)dns_zt_apply(zt, false, NULL, flush, NULL);
 	}