6672698: mangle_unused_area() should not remangle the entire heap at each collection

Maintain a high water mark for the allocations in a space and mangle only up to that high water mark.

Reviewed-by: ysr, apetrusenko

hotspot/src/share/vm/memory/dump.cpp

@@ -645,7 +645,7 @@ public:
 class ClearSpaceClosure : public SpaceClosure {
 public:
   void do_space(Space* s) {
-    s->clear();
+    s->clear(SpaceDecorator::Mangle);
   }
 };