do all caffe_copy() as UVA mem copy, and drop caffe_gpu_copy()

Do all memory copies by `cudaMemcpy` in UVA mode so that the same
`caffe_copy()` interface works for all transfers.

`cudaMemcpy()` is used in lieu of BLAS copies becuase they do not
understand UVA.

Drop the now unecessary `caffe_gpu_copy()` since location of the
pointers is now irrelevant to the interface.
replace all memcpy by caffe_copy