Restore "DstType" in AbsDiffScalarOp template arguments, even if
eventually it will be equal to SrcType

use "double" from scalar only for CV_64F input

simplify code by requesting srcDepth == dstDepth

I took the subScalar.cu code and changed the inner operation

  * modify test, too

rename cv::cuda::internal namespace
to cv::cuda::device to prevent conflicts
with cv::internal

* remove overloads with explicit buffer, now BufferPool is used
* added async versions for all reduce functions