We now need to deal with the case where a track disappears from the
labels array.

We used to destroy and recreate down streams whenever something changed,
which turned out to be racy.  We now properly implement renegotiation,
as well as atomic replacement of a stream by another one.

Webrtc 3.0.5 includes a fix for a hang in CreateOffer that may be
triggered by incorrect SDP.

The previous behaviour led to silent failures, which was confusing.

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.

We now add tracks after the stream has been pushed, so we need a lock
on down streams.  Also rework sendUpRTCP to reduce locking.

Don't break the stream, just ignore the SDP.

The default configuration is now defined by an explicit value
"auto", which is easier to check and to explain.

This can be overridden by explicitly specifying the -turn option.

This indicates that the stream is being replaced by another one with
the same id.  This avoids moving videos around when they are being
renegotiated.

renegotiate, just close the stream and open it again.

It is now more similar to the down connection, using the onclose
callback for resource management.

Make the de-facto "comment" and "contact" fields official.