We used to label tracks individually, in a view to using the labelling
for simulcast. Since then, the WebRTC community has converged on a
different strategy, where multiple tracks share a single mid and
are labelled with the rid extension.
We now label whole streams, which is simpler, and use the track's
kind (and, in the future, the rid) to disambiguate. This changes the
protocol in two ways:
* in offers, the "labels" dictionary is replaced by a single "label"
field; and
* the syntax of the "request" message has changed.
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.
