Problem with running a dns only dht

[sukunrt]

When running a dht node which advertises only /dns addresses, I see that my node is not getting any incoming connections and the outgoing connections are eventually closed as well, leading to a situation where the node has < 10 connections.

This happens because this change: #839 which added a filter for incoming addresses.
Similarly go-libp2p added a filter for addresses in identify here: libp2p/go-libp2p#2300

While both these changes are correct they relied on an incorrect version of manet.IsPublic which incorrectly classified /dns addresses as non Public. This was eventually fixed in go-libp2p and go-multiaddr by: https://github.com/libp2p/go-libp2p/pull/2553/files

There are still a lot of nodes out there on the incorrect version which is leading to /dns addresses being filtered.

1. We should add a test somewhere in this repo to prevent a regression.
2. Mention somewhere that at least on the Amino DHT, people will have to advertise their IP addresses.