vnet: Support TCP

[enobufs]

Current pion only supports UDP, but we will likely soon to support TCP as well. To help our development, we'd like to see vnet also support TCP.

See also: pion/webrtc#712

[enobufs]

@Sean-Der can we add this to the project 2.2.0? I can help.

[dgunay]

It looks like this blocks pion/ice#394

[enobufs]

TODO: look into https://github.com/xtaci/kcp-go

[stv0g]

TODO: look into https://github.com/google/gvisor/tree/master/pkg/tcpip

I think we could use the gVisor userspace networking stack to also replace the UDP transport, not only TCP.
But I am not sure to which extend the gVisor netstack also implements routing and NATting..