Verify mirror zone AXFRs

Update axfr_commit() so that all incoming versions of a mirror zone
transferred using AXFR are verified before being used.  If zone
verification fails, discard the received version of the zone, wait until
the next refresh and retry.

bin/tests/system/mirror/ns2/verify.db.in

@@ -0,0 +1,13 @@
+; Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+;
+; This Source Code Form is subject to the terms of the Mozilla Public
+; License, v. 2.0. If a copy of the MPL was not distributed with this
+; file, You can obtain one at http://mozilla.org/MPL/2.0/.
+;
+; See the COPYRIGHT file distributed with this work for additional
+; information regarding copyright ownership.
+
+$TTL 3600
+@		SOA	ns2 hostmaster 2000010100 3600 1200 604800 3600
+@		NS	ns2
+ns2		A	10.53.0.2