Remove guest rooms array; replace with a peeking SyncApi
After much discussion, the HS will now behave the same for guests/non-guests
wrt joining a room (you get the entire room state on join). This leave "peeking"
which never triggers a join. This can be implemented for guests by doing a
room initial sync followed by a specific /events poll with a specific room_id.
This means there are 2 sync streams: /sync and the peek /events. Architected
so you can only have 1 peek stream in progress at a time (if this were arbitrary
we'd quickly run into concurrent in-flight browser request limits (5).
