introduced ShenandoahTraversalUpdateRefsClosure. This is used in stw_process_weak_roots() and compared to the regular ShenandoahUpdateRefsClosure, it only checks for additional in_traversal_set(). However, in-traversal-set don't need to be updated, only in-cset that is also checked. It's reasonable to use the equivalent ShenandoahUpdateRefsClosure. This is a performance issue during the final-traversal-pause.