(Serial-) Full GC could be replaced as a series of mixed gcs.

Particularly because at the moment we collect the remembered set for all regions anyway.

Assuming that at that time this would need to be scheduled, we have already at least started marking, this marking could be completed and reused (JDK-8159705) for this "last-ditch" mixed gc phase.

This may also remove the need for explicit full gc code (at most some per-region compaction for the first few regions where we can evacuate into are needed).