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TODOs:
- need to consolidate all the versioning stuff into one package
- need to do the version check as a handshake, before further
  communication happens. we used to do this.

This addr manager should seriously help with the addrsplosion
problem.

This commit finally makes use of the sent observed addrs.
If the connection's local address is from one of our
listen addrs, then the remote's observed addr is its
natted mapping, which is useful to us. For now, we add
it directly to our address book. (a future commit should
make addressbook addresses expire)

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

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