This should hopefully fix an entire class of problems where components
downstream from the roomserver (i.e. the sync API) could just lose a
whole bunch of state after a rewrite operation like a federated join.
The root of the bug is that we set `RewritesState` in the output event
which instructs downstream components to purge their copy of any room
state, but then didn't send the entire state snapshot in
`adds_state_event_ids` so the downstream state ends up being incomplete
as a result.
fc1d8e479b