Parallelize compatibility checks

[RubenRBS]

Changelog: Feature: Parallelize compatibility checks (up to x3 speedup)
Docs: Omit

I pulled the current thread count out of thin air after testing some values locally, but as with everything having to do with concurrency, nothing is a best fit for everyone

[sykhro]

Is this tweaked by tools.build:jobs? Some of us use high core-count build machines (> 64 cores). Being able to adjust these parameters is very helpful, you never know where the bottleneck truly is

[RubenRBS]

Hi @sykhro thanks a lot for checking the PR out. After discussing it with the team, we'll start with just creating a small ThreadPool of say 8, and leave the configuration for a later date.

This is mostly because we find the UX could get a bit ugly and some questions were raised as to whether we could harmonize it with the rest of the parallel confs :)