JVMTI heap walk should consult GC for marking oops

JVMTI ObjectMark uses mark word 'marked' pattern to indicate 'visited'. That presents problem for Shenandoah GC, as it uses the pattern to indicate 'forwarding'.

Since 'marked' pattern really belongs to GC, each GC should have the say to avoid conflicts.

Therefore, I would like to purpose that the ability for each GC to provide ObjectMark, and current ObjectMarker as default implementation.