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This addr manager should seriously help with the addrsplosion
problem.

this commit moves management of the nat to its own object.
perhaps this can be general enough to work with any host
(not just BasicHost), but for now keeping here.

the nat manager:
- discovers and sets up the nat asynchronously.
- adds any port mappings necessary if/after nat has been found.
- listens to the network Listen() changes, adding/closing mappings

Exposing the NAT to the core is unnecessary. The Host can take
care of it. If a need emerges, we can address it then.

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