Use channels as a transport abstraction.

[lucaspoffo]

Experimenting with using channels as an abstraction for the transport layer. Would make easy to create new ones and would be not so complex to integrate with other crates.

This PR could be a step to implementing WebRTC #10. Also would be easy to create an Async UDP transport.

Probably should use crossbeam since the std channel is not Sync.

Renet echo example is already working.