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
Can we have a const instead of a function that tells if it is Safari? We can avoid having to detect the browser multiple times, as this fact doesn't change through the lifetime of the app.