MXS-1977: Archive session command on completion

The legacy session command implementation is still partially used and a
cleanup call was missing. This should be removed in the next major
release.

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

@@ -965,6 +965,7 @@ gw_read_and_write(DCB *dcb)
                 stmt = read_buffer;
                 read_buffer = NULL;
                 gwbuf_set_type(stmt, GWBUF_TYPE_RESPONSE_END | GWBUF_TYPE_SESCMD_RESPONSE);
+                protocol_archive_srv_command(proto);
             }
             else
             {