ENH: Implement ThreadedImageRegionPartitioner using ImageRegionSplitter

The old ThreadedImageRegionPartitioner already had the same logic as
ImageRegionSplitterSlowDimension. This commit reduces code duplication.

Deprecate some old typedefs that no longer need to be explicitly
redefined.

Change-Id: I4c9231a9862926b674791547db873edc3881a5dc
Merge branch 'master' into ImageRegionPartitioner-Refactor

Conflicts:
	Modules/Core/Common/include/itkThreadedImageRegionPartitioner.hxx

Change-Id: I41d0daa1fb207691c7ecf198afa38bc0168ad4ff
Merge topic 'ImageRegionPartitioner-Refactor'

2726e30 Merge branch 'master' into ImageRegionPartitioner-Refactor
d8e0478 ENH: Implement ThreadedImageRegionPartitioner using ImageRegionSplitter