ARROW-2455: [C++] Initialize the atomic bytes_allocated_ properly
ARROW-2455: [C++] Initialize the atomic bytes_allocated_ properly

The atomic counter `bytes_allocated_` in `CudaContextImpl` isn't initialized, leading to failure of cuda-test on windows.

Author: HE, Tao <sighingnow@gmail.com>

Closes #1892 from sighingnow/fix-initialize and squashes the following commits:

9febf78 <HE, Tao> Fix clang-format lint error.
27e4751 <HE, Tao> Initialize the atomic bytes_allocated_ properly.

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