Dual-Pivot Quicksort improvements (Radix sort)

This is a placeholder for an update of the Dual-Pivot Quicksort implementation used by java.util.Arrays sort() and parallelsort().

Proposed changes:
- fix tryMergeRuns() to better handle almost sorted datasets (all types)
- adopt radixsort() for sequential and parallel sorts on int[] / long[] / float[] / double[] arrays (almost random and length > 6K)

Vladimir Yaroslavskiy will present the changes on core-libs-dev in details.