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.
I would expect the peerconnection to report all its datachannels as closed once it's closed. If it doesn't (which we can test with an integration test), we should make it do so.

SocksToRtc doesn't currently explicitly close its sessions on termination.