SocksToRtc-style lifecycle and unit tests for RtcToNet

This pull request is to RtcToNet as the following pull request was to:
https://github.com/uProxy/uproxy-networking/pull/137

Of course, the RtcToNet "server" has only one dependency: the peerconnection. Also remove code related to control channel which is now handled by peerconnection.

Tested with `curl` and a quick browsing session on Firefox.

I will tackle RtcToNet sessions in a followup.
This worries me a little; I would expect onceClosed to complete only after the peer connection is closed, but the mock says peerConnection never closes...