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.