MXS-2562: Fix out-of-order error during COM_CHANGE_USER

If an error is generated while a COM_CHANGE_USER is being done, it would
always use the sequence number 1. To properly handle this case and send
the correct sequence number, the COM_CHANGE_USER progress needs to be
tracked at the session level.

The information needs to be shared between the backend and client
protocols as the final OK to the COM_CHANGE_USER, with the sequence number
3, is the one that the backend server returns. Only after this response
has been received and routed to the client can the COM_CHANGE_USER
processing stop.

server/modules/protocol/MySQL/mariadbbackend/mysql_backend.cc

@@ -916,6 +916,9 @@ static int gw_read_and_write(DCB* dcb)
              */
             GWBUF_DATA(read_buffer)[3] = 0x3;
             proto->changing_user = false;
+
+            auto s = (MYSQL_session*)session->client_dcb->data;
+            s->changing_user = false;
         }
     }