This commit changes the connections to use io.ReadWriters
instead of channels (+ async workers). This is a pretty
big change -- away from csp -- in the name of performance
(and predictable flow control).
It also uses the brand new secio, which is spipe's successor.
This commit actually removes the previously introduced
chan net.NetMessage, in favor of raw []byte. It plays
nicer with crypto/spipe, and it makes more sense in the
context of a "single connection", i.e. I already know the
peer I'm talking to, from the connection. The NetMessage
additional Peer is useful swarm and up.
