used BufferPool mechanism instead

they allow to perform asynchronous upload/download into temporary buffer
to get valid GpuMat object

This silences the pedantic warning messages from gcc 4.8.3

add swap instruction for allocator field

it allows to use cudaHostAlloc methods for cv::Mat objects

* it is the only place, where it is used
* no need to make this function public

move main CUDA group to modules/core/cuda.hpp

add a note to use new cudev module as a replacement

it is internal class, no need to export it

The deinitialization of BufferPool internal objects is controled by global
object, but it depends on other global objects, which leads to errors
caused by undefined deinitialization order of global objects.

I merge global objects initialization into single class, which performs
initialization and deinitialization in correct order.