* `get_forward_extremeties_for_room` takes a numeric `stream_ordering`. We were
passing a `RoomStreamToken`, which meant that it returned the *current*
extremities, rather than those corresponding to the `from_token`. However:
* `get_state_ids_for_events` required a second ('types') parameter; this meant
that a `TypeError` was thrown and we ended up acting as though there was *no*
prev state.
* `get_state_ids_for_events` actually returns a map from event_id to state
dictionary - just looking up the state keys in it again meant that we acted
as though there was no prev state. We now check if each member's state has
changed since *any* of the extremities.
Also add/fix some comments.
fc2f29c1d0