The "Rebuild free list" phase may use too many threads, making it slow.

In some benchmarks, that exhibit pause times in the range of 3-4ms, it can take around 10% of total pause.

The reason is that we seem to spawn to many threads for the little per-region work. If limited that way, it takes only 30-40% of original time.

The proposed solution is to merge this phase with post evacuation phase 2.