The separation of work in the p2p pkg is as follows:

- net implements the Swarm and connectivity
- protocol has muxer and header protocols
- host implements protocol muxing + services
- identify took over handshake completely! yay.
- p2p package works as a whole

using a placeholder net2 package so tests continue to pass.
Will be swapped atomically into main code.

remove comment about spdystream. this is now
superceded by go-peerstream interface.

The net package is the next to move. It will be massaged
a bit still to fix the Network / "NetworkBackend" conflict.

The crypto package moves into p2p. Nothing in it so far is ipfs
specific; everything is p2p-general.

I think it's time to move a lot of the peer-to-peer networking
but-not-ipfs-specific things into its own package: p2p.
This could in the future be split off into its own library.
The first thing to go is the peer.