introduce a proper address pipeline

[marten-seemann]

We need to get a lot smarter about address management. Wrapping / replacing addresses via the AddrsFactory doesn't scale.

I'm imagining a new service that's responsible for the entire process of address management.

1. It ingests in new "address candidates" from various sources: user provided address hints, transports claiming to listen on certain addresses, Identify, relay service etc.
2. It verifies these addresses using AutoNAT v2.
3. It groups and categorizes these addresses (public / private / relayed).
4. It provides interfaces to make these different categories of addresses available to consumers.

Updated(@sukunrt): Using this issue for tracking:

• Implement AutoNATv2 #2422

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1. autorelay: send addresses on eventbus; dont wrap address factory #3071
   +0
2. addressservice: extract out addressing logic from basichost #3075
   +0
3. Tag the source of the address with the address
4. Add a address reachability verifier module
5. Introduce new event to send address updates along with address's reachabilty
6. Separate address service in to its own package
7. move observed addr manager to new package

[sukunrt]

We need something like #2293 to test this.

[sukunrt]

The flow here is:

1. Get listen addresses from the swarm.
2. Different modules take these listen addresses and provide an estimate of the external address corresponding to these listen addrs.
3. Collect all this info and test them via autonat v2.

So we have an object that's something like:

type AddrDetail struct {
    ListenAddr ma.Multiaddr // Swarm listen addr
    InterfaceAddr ma.Multiaddr // The interface for this address
    ExternalAddr ma.Multiaddr // The public facing address for this address
    Sources map[Source]struct{} // source can be machine interface, Identify, UserProvided(addr factory?), NAT Port Map, Autorelay
}

We then test these via autonat v2 and send updates on eventbus that includes both new addresses and their reachability changes.

[sukunrt]

https://github.com/libp2p/go-libp2p/pull/3075/files

Refactors the existing logic enough to make it readable.