RFR: Merge marked_object_iterate() impls

[Roman Kennke]

This merges the two almost identical marked_object_iterate()
implementations in ShenandoahHeap vs. ShenandoahHeapRegion.

It introduces two new methods marked_prev_object_iterate() and
marked_next_object_iterate() which both call the private helper
marked_object_iterate() with the corresponding mark bitmap and
top_at_mark_start.

http://cr.openjdk.java.net/~rkennke/marked_object_iterate/webrev.00/

Tested by hotspot_gc_test.

Ok?

Roman