Summary:
PgClient relies on heartbeat mechanism to cleanup postgres session that were closed.
But it takes significant amount of time to realise that connection was actually closed to release resources.

This diff introduces new logic to quickly release pg client session when TCP connection was closed.

Upgrade/Rollback safety: Persistent format or exchange format between different software versions was not changed.
Jira: DB-13951

Test Plan: PgSharedMemTest.ConnectionShutdown

Reviewers: hsunder

Reviewed By: hsunder

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D39954