GTM Standalone connectivity improvement (2)

Today, I finished the first code for this.   Tested the following:

1. start gtm
2. start gtm_proxies
3. start coordinators/datanodes and do some session through psql.
4. start gtm standby and do another psql sessions.

They worked okay.  Promote and reconnect seems to work fine.

However, when gtm standby is shutdown and try to connect again, gtm/gtm_proxies seems to stall.   Maybe disconnect sequence doesn't work well.  Need to fix this before commit.