use_token: update only when addresses differ, and update serial.

src/storage.cr

@@ -338,6 +338,9 @@ class DNSManager::Storage
 		zone = zone_must_exist! token.domain
 		rr = zone.rr_must_exist! token.rrid
 
+		# Same address, no need to change anything.
+		return Response::Success.new if rr.target == address
+
 		# TODO: validate target?
 
 		case rr
@@ -349,7 +352,7 @@ class DNSManager::Storage
 		when Zone::AAAA
 			rr.target = address
 			zone.update_rr rr
-			zones_by_domain.update_or_create zone.domain, zone
+			update_zone zone
 			Response::Success.new
 		else
 			Response::Error.new "use token on invalid entry (not A or AAAA)"