fix(@libp2p/webrtc): close data-channel on muxer stream end

[Marcel-G]

#1895

Problem

RTCDataChannels are not properly closed after the stream ends. They stay in an open state and eventually the browser will not be able to create more RTCDataChannels with the following error:

Uncaught (in promise) DOMException: Failed to execute 'createDataChannel' on 'RTCPeerConnection': RTCDataChannel creation failed

Solution

Close data channels when the stream ends.

[achingbrain]

CI has become very sad - could this be because the WebRTC data channel is now being closed before any queued data has been sent?

This is usually what the "Error: stream ended before 1 bytes became available" error means.

[marcus-pousette]

Would be great if this could be part of next release. This would perhaps also solve a related issue

[achingbrain]

This PR needs a followup:

1. Perhaps close the datachannel in the receiver as well for safety
2. Add tests that assert we can open/close a large amount of streams
3. Maybe figure out how to assert that WebRTC DataChannels are closed correctly - they are scoped to the calling context so they'll need to be exposed externally somehow and cleaned up from however they are exposed