If we returned multiple distinct ranges, we always assumed that
the history visibility was "joined", so we would never return
events in the invite/shared state. This would be fine if the client
had a way to fetch those events sent before they were joined, but
they did not have a way as the prev_batch token would not be set
correctly. We now only return a single range of events and the prev
batch for /that/ range only, and defer to the upstream HS for
history visibility calculations.
Add end-to-end test to assert this new behaviour works.
