JDK-8350441 was implemented before IntrusiveRBTree was introduced, and as such implemented its own intrusive red-black tree. Now that a shared data structure implementation is available, use that instead.

The switch is straight forward, and the O(1) left and right most node lookup which ZIntrusiveRBTree implements that IntrusiveRBTree does not is trivial to implement on top of the tree.

Initial performance evaluation shows no difference between the two implementations. And the functional testing passes.