Providing unknown addresses to the user, helps them infer whether autonatv2 has any more addresses it might confirm.
There's no upside to not sending this information.
This also changes host.ConfirmedAddrs to return all the three categories, Reachable, Unreachable, and Unknown addrs.
This introduces addrsReachabilityTracker that tracks reachability on
a set of addresses. It probes reachability for addresses periodically
and has an exponential backoff in case there are too many errors
or we don't have any valid autonatv2 peer.
There's no smartness in the address selection logic currently. We just
test all provided addresses. It also doesn't use the addresses provided
by `AddrsFactory`, so currently there's no way to get a user provided
address tested for reachability, something that would be a problem for
dns addresses. I intend to introduce an alternative to
`AddrsFactory`, something like, `AnnounceAddrs(addrs []ma.Multiaddr)`
that's just appended to the set of addresses that we have, and check
reachability for those addresses.
There's only one method exposed in the BasicHost right now that's
`ReachableAddrs() []ma.Multiadd`r that returns the host's reachable
addrs. Users can also use the event `EvtHostReachableAddrsChanged`
to be notified when any addrs reachability changes.
