Scaling: Get rid of maxValidators configuration

[eskimor]

Vision

We started parachain consensus with a maxValidators configuration parameter. This was meant as a intermediate step in order to be able to gracefully scale up the number of validators participating in parachain consensus. Now the time has come where we should work on being able to remove that constant. For this we need to:

The Plan

1. Find out exactly where current bottlenecks are
2. Create a concrete list/tickets of those bottlenecks (discovered via metrics, experiments and profiling)
3. Fix those bottlenecks in order of most expected impact/ease of fixing
4. Scale up the number of parachain validators until we reached the full set.
5. Get rid of maxValidators

This is a first step in enhancing parachain consensus performance, once we got rid of maxValidators we will focus on scaling up the number of parachains as well. Work on this ticket will already have an impact on that number obviously, but we start with a focus on increasing maxValidators and only after we got rid of that constant, we will look into further optimizations to increase the number of parachains (if necessary).

Board to keep track of this project: https://github.com/orgs/paritytech/projects/63/views/1

Bottlenecks:

• approval-distribution: batch assignment and approval vote send polkadot#6400