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.
As discussed in person, let's do this here and in SocksToRtc in a followup pull request. I added a TODO and filed an issue:
https://github.com/uProxy/uproxy/issues/561