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This commit makes all network tests use ZeroLocalTCPAddress
as the initial peer address, and then relies on
net.ListenAddresses()

This should get rid of the tcp addr clash problems.

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superceded by go-peerstream interface.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.