The server now maintains a set of statuses for each user that are not
interpreted by the server but communicated to the other members of the
group using 'user' messages.
We used to signal connection replacement by reusing the same connection
id. This turned out to be racy, as we couldn't reliably discard old
answers after a connection id was refused.
We now use a new id for every new connection, and explicitly signal
stream replacement in the offer message. This requires maintaining a
local id on the client side.
