Parallel: Decouple maximum compaction from SoftReference clearing

SoftReference clearing controls whether a SoftReference should be treated as strong-ref or weak-ref.

Maximum compaction controls whether space occupied by some dead objs should not be reclaimed to avoid moving following objs (in a sliding scheme).

The former acts during marking while the latter after marking.

The current implementation mixes the two concepts. Decouple them to avoid confusion and offer more freedom of control.