#484 pgbench still fails in the initialization phase with a GTM error

[cbx]

pgbench still fails in the initialization phase (creating tables and inserting data) with an error "GTM error, could not obtain snapshot".
In sheer datum, the error is occur even if single node and small scale factor, increase of the node also increase rate of incidence. And setting scale factor over 5000 seems to raise the rate sharply.

This issue can be modeled to the following operation. The next graph shows connections from back-end to GTM Proxy and transactions, and the life cycle of each connection is the same as related transaction's one: the left end indicates the connection established and the transaction began, the right end indicates the transaction finished and the connection disconnected.

Connection 1 / Transaction 1: |----------------------------|
Connection 2 / Transaction 2:                        |-----------------------------|
Connection 3 / Transaction 3:                                             |-----|

After connection 3 disconnected, Transaction 2 disappeared from GTM and any operation on Transaction 2 fails.

Commit 5feb1fa63d93ba40ebfdef097fde347cecdd8a3a fixed this issue.