Hold a catz reference while the update process is running

This should delay the catalog zone from being destroyed during
shutdown, if the update process is still running.

Doing this should not introduce significant shutdown delays, as
the update function constantly checks the 'shuttingdown' flag
and cancels the process if it is set.

lib/dns/catz.c

@@ -2124,6 +2124,7 @@ dns__catz_timer_cb(void *arg) {
 		      ISC_LOG_INFO, "catz: %s: reload start", domain);
 
 	dns_catz_ref_catzs(catz->catzs);
+	dns_catz_ref_catz(catz);
 	isc_work_enqueue(catz->loop, dns__catz_update_cb, dns__catz_done_cb,
 			 catz);
 
@@ -2543,6 +2544,7 @@ dns__catz_done_cb(void *data) {
 		      ISC_LOG_INFO, "catz: %s: reload done: %s", dname,
 		      isc_result_totext(catz->updateresult));
 
+	dns_catz_unref_catz(catz);
 	dns_catz_unref_catzs(catz->catzs);
 }