net-io

(after #442)

This PR changes net to use `io` instead of channels.
At the moment it's just `conn`, but soon more.
Yeah. I tried to integrate this mock into the bitswap mock but couldn't.

We'd get immediate returns on investment if this mock worked that way. Immediately, we could use the IpfsDHT in bitswap+network+dht tests. The tests would be very reliable. We'd setup topologies and find problems pretty quickly.

:gift: :christmas_tree: :gift: