Show display name and avatar on messages

[bwindels]

We need to store either the most recent m.room.member eventId or the display name and avatar on an incoming event. We can have a FILO cache of x members for synced messages, and load a state event from store if it's not in there. But how do we handle filling gaps? How do we find out what the room state was at the beginning of the fragment we're trying to backfill (end, forward-fill, respectively).