Support for different relay selection strategies

[PaulOlteanu]

It would be nice to be able to customize the relay selection strategy. For example, to randomly select from the best n by latency rather than just the single one with lowest latency.

This would be useful when running many smaller-capacity relays to ensure clients are distributed (somewhat) evenly across them rather than overloading a single one

[matheus23]

Another idea that has been floated internally was to have relays be aware of their current load, and if exceeding a threshold, redirect clients to another node.

That said, this may require some coordination between relays.