Proposal: Remove stopped transceivers to prevent resource exhaustion

Howdy WebRTC enthusiasts.

Issue: Remove stopped transceivers from getTransceivers()
<https://github.com/w3c/webrtc-pc/issues/2092>

TL;DR: Keeping transceivers around prevents garbage collection of a
possibly ever-growing list of tracks. Can we stop doing that?

If an application does addTrack() and removeTrack() with renegotiations
in-between, the list of m= sections and transceivers will grow over time.
RTCRtpTransceiver.stop() allows negotiating that an m= section is no longer
used, and in subsequent renegotiations that m= section could be recycled
for future transceivers preventing the SDP from growing indefinitely.
However, currently the set of transceivers
<https://w3c.github.io/webrtc-pc/#transceivers-set> (getTransceivers()) is
never cleaned up and will grow indefinitely.

This prevents the transceivers, and thus the associated senders/receivers
and tracks, from being garbage collected. A stress test running Chrome that
did add/removeTrack a lot ran out of memory when we switched from Plan B to
Unified Plan due to this resulting in a few thousand transceivers in less
than a minute. (I don't know the true footprint, but even if each
transceiver just has a single video frame in its buffer of 1280x720x4,
that's several megabytes, multiplied by thousands of transceivers =>
gigabytes of data).

*Shall we make stopping a transceiver remove it from the connection's set
of transceivers?*

Note: It is still possible to prevent the list of transceivers from growing
by *manually* recycling them using transceiver.sender.replaceTrack() and
transceiver.direction, but that still wastes resources on transceivers
currently not used, and implies you probably shouldn't use
transceiver.stop() in most cases.

Related issues/questions:

   1. If you care about mids of stopped transceivers you'd have to record
   them yourself.
   2. There is no "onstopped" event in case a transceiver is stopped due to
   setRemoteDescription(). Should we add one, or is it enough that the
   application checks whether the transceiver still exists by querying
   getTransceivers() in track.onmute?

Please discuss.