MXS-1203: Better handling of batch queries

When batched queries are done through readwritesplit, it will now handle
them one by one. This allows batched queries to be used with
readwritesplit but it does impose a performance penalty when compared to
direct execution on the backend.

include/maxscale/protocol/mysql.h

@@ -430,6 +430,8 @@ bool mxs_mysql_is_ok_packet(GWBUF *buffer);
 /** Check for result set */
 bool mxs_mysql_is_result_set(GWBUF *buffer);
 
+/** Get current command for a session */
+mysql_server_cmd_t mxs_mysql_current_command(MXS_SESSION* session);
 /**
  * @brief Calculate how many packets a session command will receive
  *