Summary:
The YB_TODO in postinit.c mentions possible merge conflicts. After reviewing the
YB changes, I found they are fine because they are either
(1) new YB specific code inserted, or
(2) PG code skipped in YB environment.

I renamed the YB argument `session_id` to `yb_session_id`. Also added one
additional `if (IsYugaByteEnabled())` to make YB specific code a bit clearer.
Jira: DB-14689

Test Plan: jenkins

Reviewers: kfranz, mihnea

Reviewed By: kfranz

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D40930