[lib] Introduce MessageSpec.getLastUpdatedTime

Summary:
This new method on the MessageSpec will allow us to address ENG-9558. If it returns null or undefined that means that the message should not bump the thread's timestamp. If it returns a Promise that means we don't yet know if it should bump the thread's timestamp.

For now, in this diff we're ignoring any messages whose MessageSpec.getLastUpdatedTime returns a Promise, and assuming it's the same as if it returned null. In later diffs we'll update the logic to be smarter.

Depends on D13912

Test Plan: Tested in combination with the rest of the stack. See video in the last diff

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13913