In order to make sure that startReached starts out as true when a thread is created, we set truncationStatus to "exhaustive".

I thought about always using exhaustive, but this conflicts with our plans in the Limit number of displayed DMs project, where we'll want startReached to be false if there are messages in SQLite that are not represented in Redux.

Depends on D12827

I followed the test plan described in this Linear comment. Before this diff, the spinner was visible in the newly-created thick thread. After this diff, it was no longer visible