- does end properly
- no goroutines leaked!

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.

This Commit changes the relationship between Conn and Swarm.
After this, Conn is significantly more autonomous, and follows
an interface.

From here, it will be very easy to make the MultiConn (that
handles multiple Conns per peer).

@whyrusleeping `make vendor` or travis fails

(that wasn't mine :p)

Otherwise don't have the peer's target address.

Important bugfix. Otherwise bitswap cannot message peers
the node has not connected to yet :(