loadbalancer-experimental: add support for prioritization

[bryce-anderson]

Motivation:

We want a way to support grouping endpoints into prioritization
groups so we can favor some, but still fall back to another group
in the event things go wrong.

Modifications:

Add the notion of prioritization. This lets users define groups
which then get translated into adjusted weights and fed into
the standard endpoint selection algorithms.

[tkountis]

Missing a few finals here & there, but nothing of importance to comment on, looks promising.
It would be nice to see how a sub-setting concept would look in this implementation. I have to process it a little more in my head, but i imagine the weight becomes very overloaded, and my fear is that reasoning will be harder.

[bryce-anderson]

Missing a few finals here & there, but nothing of importance to comment on, looks promising. It would be nice to see how a sub-setting concept would look in this implementation. I have to process it a little more in my head, but i imagine the weight becomes very overloaded, and my fear is that reasoning will be harder.

You're right: using weight will make things more overloaded. Tbh, the overlay of subsetting and priority is probably going to be very tough regardless of how we represent them and are probably only compatible with some relatively stringent constraints.