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This commit cleans up the reuse port setup, and fixes a problem:
make sure to filter addrs out that we simply cannot dial with
(e.g. loopback -> non-loopback, or linklocal -> nonlinklocal)

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)

cc @jbenet @whyrusleeping

Good thing, i checked one wrong, too!

the travis docs say the env flag TRAVIS=true but, i want
to make absolutely sure. explicitness leaves less room for
error.

p2p/net/swarm: more connection bugs

this is actually pretty important in case some peers end
up changing addrs, so others dont fail dialing simply for
picking the wrong addr to start with.

This commit introduces a backoff when failing to dial
peers. It makes everything much faster.

* filtering InterfaceListenAddresses(s) instead
* return error if for loops skip
* large dial timeout