RFR: Cleanup buffered queue handling for Shenandoah

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/concmark-queue-cleanup/webrev.01/

This cleans up the handling for our marking queues: encapsulates most of the code in the queue code
itself, and moves our new implementation to our own file, reducing upstream exposure. We can move
new implementations back after upstreaming.

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey