Server relay support

[JeDaYoshi]

This would be something interesting to do an implementation of.

I have an issue where sometimes peers cannot connect to each other (due to provider-level issues mostly, but I guess firewall situations could happen here too), so something that could help are peers that can serve as a relay (basically, instead of peer1 -> peer2, peer1 -> peer3 -> peer2) to help mitigate these issues.

Tailscale has a solution for this, using their own relay servers. It'd be nice to see something similar on innernet (but maybe not going too as like using TCP, at least for a start, since... that'd surely be a fun thing to do).

[mcginty]

This is definitely something that is worth adding, totally agree. I think the general strategy as of now is to fix some more basic NAT traversal issues (tracking in #109), and then move on to adding server relays while considering adding support for site-to-site type relays as well.