Event-based autonat checking

[Stebalien]

Instead of using timeouts to determine when we should next try to check our NAT status, if we don't know our status:

1. Listen for identify events for new peers.
2. If the new peer supports AutoNAT try them.

And keep doing this till we find out our status.

[willscott]

should react to protocol updates rather than identify events once protocol updates are emitted reliably - per libp2p/go-libp2p#860

[marten-seemann]

I think that's what we're doing:

go-libp2p-autonat/autonat.go

Lines 202 to 208 in f9d8855

	case event.EvtPeerIdentificationCompleted:
	if s, err := as.host.Peerstore().SupportsProtocols(e.Peer, AutoNATProto); err == nil && len(s) > 0 {
	currentStatus := as.status.Load().(autoNATResult)
	if currentStatus.Reachability == network.ReachabilityUnknown {
	as.tryProbe(e.Peer)
	}
	}