RFR 8003981: Support Parallel Array Sorting - JEP 103

[Chris Hegarty]

This is a review request for the addition of utility methods to 
java.util.Arrays that provide sorting of arrays in parallel, JEP 103 [1].

Webrev:
   http://cr.openjdk.java.net/~chegar/8003981/ver.00/

Current sorting implementations provided by the Java Collections 
Framework (Collections.sort and Arrays.sort) all perform the sorting 
operation sequentially in the calling thread. This enhancement will 
offer the same set of sorting operations currently provided by the 
Arrays class, but with a parallel implementation that utilizes the 
Fork/Join framework. These new API’s are still synchronous with regard 
to the calling thread as it will not proceed past the sorting operation 
until the parallel sort is complete.

The actual sorting API this proposal adds will leverage the 
ForkJoinPool.commonPool (default Fork/Join pool defined as part of JEP 
155). The sorting algorithm is that used in Doug Lea’s ParallelArray 
implementation.

This work was originally done over in the lambda/lambda repo by David 
Holmes, and is now being cleaned up and brought into jdk8.

Open issues/differences from version in lambda:
1) Is is necessary for MIN_ARRAY_SORT_GRAN to be in the public API?
    It is an implementation detail (easy to remove).
2) The use of FJP.commonPool is an implementation detail, not
    part of the spec. Should not be a problem, just worth pointing
    out, as it differs from what is in lambda/lambda.

-Chris.

[1] http://openjdk.java.net/jeps/103