Make a deep copy of queued queries

If a shallow copy of the buffer is made, any modifications that are made
to the data after it has been queued will affect the queued query of the
LocalClient.

A copy-on-write mechanism would save the relatively expensive process of
copying the data but since the LocalClient is not often used, it is not
the most critical performance problem.

server/modules/protocol/MySQL/mariadb_client.cc

@@ -53,7 +53,7 @@ bool LocalClient::queue_query(GWBUF* buffer)
 {
     GWBUF* my_buf = NULL;
 
-    if (m_state != VC_ERROR && (my_buf = gwbuf_clone(buffer)))
+    if (m_state != VC_ERROR && (my_buf = gwbuf_deep_clone(buffer)))
     {
         m_queue.push_back(my_buf);