MXS-2196: Replace listener iterator

The iteration of listeners is now done via the global list of
listeners. This removes the need to have a service before a listener is
accessed which also reflects how the actual configuration is laid out. It
also guarantees that any results returned by the find functions will be
valid as long as the results are used.

server/modules/protocol/MySQL/mariadb_client.cc

@@ -266,10 +266,8 @@ LocalClient* LocalClient::create(MYSQL_session* session, MySQLProtocol* proto, c
 LocalClient* LocalClient::create(MYSQL_session* session, MySQLProtocol* proto, SERVICE* service)
 {
     LocalClient* rval = NULL;
-    LISTENER_ITERATOR iter;
 
-    for (SERV_LISTENER* listener = listener_iterator_init(service, &iter);
-         listener; listener = listener_iterator_next(&iter))
+    for (const auto& listener : listener_find_by_service(service))
     {
         if (listener->port > 0)
         {