Summary:
We use individual thread for communication via shared memory with postgres.
As result new thread is started for each new postgres connection.
Could use thread pool to reuse threads between postgres connections.
Jira: DB-15867

Test Plan: PgSharedMemTest.ConnectionShutdown

Reviewers: rthallam, esheng

Reviewed By: esheng

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D42710