6883834: ParNew: assert(!_g->to()->is_in_reserved(obj),"Scanning field twice?") with LargeObjects tests Summary: Fixed process_chunk_boundaries(), used for parallel card scanning when using ParNew/CMS, so as to prevent double-scanning, or worse, non-scanning of imprecisely marked objects exceeding parallel chunk size. Made some sizing parameters for parallel card scanning diagnostic, disabled ParallelGCRetainPLAB, and elaborated and clarified some comments. Reviewed-by: stefank, johnc

// I can cast any old thing I want to this type object:
public class You_Have_Been_P0wned {
  // Make a bunch of int-fields so I can peek & poke it
  int _a;
  int _b;
  int _c;
  int _d;

}