Sliding Sync: Track whether we have sent rooms down to clients (#17447)

The basic idea is that we introduce a new token for a sliding sync
connection, which stores the mapping of room to room "status" (i.e. have
we sent the room down?). This token allows us to handle duplicate
requests properly. In future it can be used to store more
"per-connection" information safely.

In future this should be migrated into the DB, so its important that we
try to reduce the number of syncs where we need to update the
per-connection information. In this PoC this only happens when we: a)
send down a set of room for the first time, or b) we have previously
sent down a room and there are updates but we are not sending the room
down the sync (due to not falling in a list range)

Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>