Verify that addresses reported by identify are correct before inserting them in the DHT

[tomaka]

Right now nodes tell us the addresses they are reachable from, and we immediately insert them in the DHT without any verification. This leads to a lot of unreachable addresses in the DHT, such as 127.0.0.1 or IPs behind NATs.
Instead, we should first attempt to connect to these addresses to see if they're reachable.

Note that connectivity is not transitive. For example if a node A is listening on a given address, this address might be reachable by B but not by C. If we implement this change, then C will not tell B about that address that A has, even though it would have been desirable. This is considered an acceptable trade-off.

On the implementation side, the good news is that Kademlia should automatically detect us reaching the node and add the address we connected to to the DHT, without having to call any method (although I'm not sure that this is the case, but I think that it should be the right logic).

In other words, all we have to do is call Swarm::dial for each address that a node reports through identity.

However, this is complicated by the "one connection per node" policy. If we connect to a node we're already connected to, we're going to drop the existing connection.
In other words, we first have to land libp2p/rust-libp2p#1440

cc @romanb @arkpar

[twittner]

We have to be careful though not to introduce a DDOS vector by just dialling every reported address. Rate-limiting the dialling attempts may help a bit.

[tomaka]

Even after #563 there is still a problem to solve:

At the moment we insert nodes in the DHT as soon as we connect to them. We should instead only insert in the DHT nodes that belong to the correct chain.

In other words, we should wait until we know that node is on chain A before inserting it in the DHT of chain A.

See also paritytech/substrate#3303