Remove _scan_top from HeapRegion

HeapRegion's _scan_top serves the single purpose that during garbage collection we do not try to scan areas that are potentially being allocated into at the moment.

This enhancement suggests to remove _scan_top from HeapRegion and take a snapshot of top() values at start of GC.

This removes potentially slow access to top/scan_top during GC as we need to serialize using a loadload barrier.

Additionally, the "new" scan_top information can be subsumed with the cascade of if's that refinement during GC needs to do otherwise.

Incidentally doing all that improves update rs/scan rs times in benchmarks.