Add ForkJoin common pool and CountedCompleter

As part of JEP 155: Concurrency updates (jsr166e), this issue proposes to update ForkJoin related classes, and add a common FJ pool.

"Added functionality and improved performance for ForkJoinPools that allow them to be used more effectively in the increasingly wider range of applications that users desire. New features include support for completion-based designs that are often most appropriate for IO-bound usages, among others."