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