magic_endpoint: allow dialing with just PeerId

[flub]

The magic endpoint stores known addresses of peers in the PeerMap. This should allows it to keep state across dials and allows successful dials with just a PeerId. We should allow this.

See #1222 (comment)

[rklaehn]

I think it is very important that we enable this somehow, soon.

For me this is one of the main selling points of iroh-net. It is very simple to explain: you get all of QUIC, but you can dial by identity (peer id) instead of location (addr). Basically QUIC but p2p, very good and short elevator pitch.

So why can't we have some (non-persisted) information exchange between the derpers that allow them to figure out a peer => current derp region mapping?

A peer knows or at least can find out its own region id. So when it connects to its derp region, this information could be distributed between the derpers, no?

At least for me this is extremely valuable to make iroh-net a compelling product that people might be willing to pay for.

[rklaehn]

Can't we just hack dial by peer in the most non decentralized way possible? Have a single central database that stores peer -> region, that gets updated every time a peer connects to a derper, and gets queried every time we need to know where a peer is?

Once we go via the relays we are in a somewhat centralized world anyway. So I say let's just hack it for now and then come up with something more pure/decentralized later...