Add Scalable Updatable Variables, DoubleAccumulator, DoubleAdder, LongAccumulator, LongAdder

As part of JEP 155: Concurrency updates (jsr166e), this issue proposes to add Scalable Updatable Variables. From the JEP:

"Scalable updatable variables. Maintaining a single count, sum, etc., that is updated by possibly many threads is a common scalability problem. A small set of new classes (DoubleAccumulator, DoubleAdder, LongAccumulator, LongAdder) internally employ contention-reduction techniques that provide huge throughput improvements as compared to Atomic variables. This is made possible by relaxing atomicity guarantees in a way that is acceptable in most applications."