fix(iroh-net): reverse ip-port mapping stores only direct addresses in the peermap

[divagant-martian]

Description

We were storing reverse mappings for SendAddr::Derp in the PeerMap. This means basically treating the DERP as a peer itself. We concluded this is something we shouldn't do. To fix the issue, the better solution is to enforce it via the type system. As such, we now only do reverse mapping from socket addresses.

As a note, I added a new type IpPort. Using SocketAddrs as key to hashmaps often goes wrong with ipv6 because of the flow control & class fields it contains. These fields can't be assumed stable even within the same connection: sending a the socket address made from an (ipv6, port) and receiving a response can make it look like the socket address is different (same ip, same port, different flow labels) Ignoring these fields has some limitations in extremely exceptional cases (link local addresses with a single interface) As a note, ignoring these fields is what tailscale does (all they store is an ip and a port). This makes communications over ipv6 work better in the common case so I think it's worth doing

Notes & open questions

Some references

• flow label RFC 2460 section 6
• traffic class RFC 2460 section 7
• scope id RFC 4007

Change checklist

• Self-review.
• Documentation updates if relevant.
• Tests if relevant.