GTM standby

Now GTM standby is at the last stage.

1. Error handling in GTM-Proxy done: when GTM communication error occurs, proxy does some retry and if it fails then wait for reconnect operation.
2. GTM standby can now connect to GTM at any moment.
3. After reconnect and shutdown whole cluster, GTM-Standby can start as GTM.   Do not forget to rearrange configuration files properly.
4. GTM backup is now corrected so that transaction handle and GXID are backed up to the standby.
5. Asynchronous backup works fine.
6. Synchronous backup has still some issues.   Maybe some minor protocol handling.   Will be tested next.