Summary:
It could happen that pg client session is destroyed while `big_shared_mem_expiration_task_` is scheduled to run.
As result it could be executed on destroyed object.
Jira: DB-13158

Test Plan: Jenkins

Reviewers: rthallam, esheng

Reviewed By: esheng

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D38688