Segregate routable and non-routable networks

Nobody has claimed this yet.

Currently, we announce *all* IP addresses we're listening, even private ones, and will *dial* any address we find for a peer, even private ones. We do this intentionally so we can connect to nodes on our local network. Unfortunately, this causes issues like https://github.com/ipfs/go-ipfs/issues/5511 and really pisses off users (for obvious reasons).

One reasonable solution (IMO) is to segregate routable and non-routable networks. That is:

1. If we learn about a non-routable IP address from a routable network, discard it.
2. If we know a non-routable IP for a peer, only distribute it to peers with non-routable addresses.

We'll still be able to discover and dial nodes on non-routable networks through local discovery (mDNS). Additionally, if we're running IPFS on a VPN and *all* peers have non-routable addresses, everything should "just work".

This will mostly require changes to the DHT and the Identify protocol.

Future work:

* https://github.com/libp2p/notes/issues/3
* Separate DHTs? We could have a private one and a public one.

Start by reading the DHT and Identify protocol implementations, tracing how peer addresses are learned, advertised, and dialed. Done means non-routable addresses learned from routable networks are discarded or restricted to peers with non-routable addresses, while local discovery and all-non-routable VPN scenarios continue to work.

Written by the indexing model from the issue text.