ARROW-3516: [C++] Use unsigned type for difference of pointers in parallel_memcpy
ARROW-3516: [C++] Use unsigned type for difference of pointers in parallel_memcpy

Author: Kouhei Sutou <kou@clear-code.com>

Closes #2760 from kou/cpp-fix-memcpy-size-type and squashes the following commits:

14c59bad2 <Kouhei Sutou>  Fix memcpy's size type

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