deprioritize unspecified addresses in mock connections

[willscott]

slightly less naive address selection.

[Stebalien]

LGTM but really, we should just resolve unspecified addresses at a higher layer.

[willscott]

The underlying issue is that .Addrs() has an overloaded meaning: if i want to know where i'm listening / what my potential source addresses are, the unspecified address is sometimes relevant. if it's in this context / in terms of addresses i'm announcing to others, than it stop being meaningful.

[Stebalien]

If you want to know where you're listening, you usually use host.Network().ListenAddresses(). peerstore.Addrs(peer) should return dialable addresses.