ES: Add combining strategy

[ben-manes]

The EliminationStack is able to cancel opposing operations (push - pop). This works well in a balanced workload to reduce contention on the stack.

In an unbalanced workload, the producers contend and the stack degrades to its original form. There is also a penalty of scanning the arena for a potential mate. If combining is supported, then both of these problems are eliminated.

http://www.cs.bgu.ac.il/~hendlerd/papers/DECS.pdf

[ben-manes]

Added in 7772145