[lib] Consider parent timestamp in lastUpdatedTimeIncludingSidebars

Summary:
I made a mistake in D14154: I forgot that we should always consider the parent thread's timestamp in lastUpdatedTimeIncludingSidebars.

Depends on D14156

Test Plan: Confirm that we no longer see a massive performance regression

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D14157