G1: Record regions where evacuation failed to provide targeted iteration

At the end of gc we need to iterate over all regions where evacuation failed to fix up self-forwarded pointers.

Currently, particularly after JDK-8254164, we iterate over all collection set regions in parallel with all threads.

This is a problem with current work distribution: since we do not know how many regions with actual work there are, we spin up all available threads, with no idea

- which thread gets to do which region.
- how much work is actually there, causing issues with synchronization of spinning up and down lots of threads unnecessarily