add socket-load-balance flag

[graingert]

Summary

Fixes #2467

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[graingert]

Needs python/cpython#124761

[Kludex]

Why do we need to add a new argument i.e. socket-load-balance? Can't this behavior be the default? I'd assume people already thinks that's the default.

[graingert]

There's a tradeoff where the default unfair load balancing favours lower latency and the SO_REUSEPORT(_LB) approach favours an even load between processes

I suspect most people do want the fair load balancing, however I think it should bed in a bit before we suddenly turn it on by default for everyone.