NAT traversal: ICE-esque candidate selection

[mcginty]

This change adds the ability for peers to report additional candidate endpoints for other peers to attempt connections with outside of the endpoint reported by the coordinating server.

While not a complete solution to the full spectrum of NAT traversal issues (TURN-esque proxying is still notably missing), it allows peers within the same NAT to connect to each other via their LAN addresses, which is a win nonetheless. In the future, more advanced candidate discovery could be used to punch through additional types of NAT cone types as well.

Related discussions #109 and #115

[strohel]

I've done an initial review, found one likely oversight and a few nits. I'll wait until this is not marked Draft to do a more thorough review and perhaps some testing?

Meta: the fact this was split to more or less incremental commits helped with review. Commits like 23f391b client: break out peer diff printing to function for readability that are strictly "just move stuff around" or strictly "add new functionality" are the best. (OTOH the very same function is moved again in f560598) I still don't have a good idea what the best balance between prototyping friendliness and review friendliness is.

[strohel]

Looks good, I was finally able to grasp this in its entirety! Only the netlink response parsing code is somewhat gnarly, but that's likely due to inherent complexity of the netlink API itself.

I've added a few nits/suggestions.

[pri11er]

I was just giving this a try, going from 1.4.1, and server fails to start:
innernet-server[10422]: INFO innernet_server::db > migrated db version from 1 to 2
innernet-server[10422]: Error: internal database error
innernet-server[10422]: Caused by:
innernet-server[10422]: Invalid column type Null at index: 10, name: candidates
.service: Main process exited, code=exited, status=1/FAILURE

Revert and it starts ok again:
innernet-server[10561]: INFO innernet_server::db > migrated db version from 2 to 1
innernet-server[10561]: INFO innernet_server > bringing up interface.
innernet-server[10561]: INFO innernet_server > 12 peers added to wireguard interface.
innernet-server[10561]: INFO innernet_server > innernet-server 1.4.1 starting.

[mcginty]

You're on top of things @pri11er :).

This is fixed in #136.