net-io

(after #442)

This PR changes net to use `io` instead of channels.
At the moment it's just `conn`, but soon more.
If so, this might be more natural.
```Go
    LinksBetweenPeers(peer.Peer, peer.Peer) []Link
```

If it's all of the potential underlying network connections, then it might be too much information/break in abstraction.