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ARROW-1850: [C++] Use void* / const void* for buffers in file APIs

This makes for a more convenient / less rigid API without as need for as many usages of `reinterpret_cast<const uint8_t*>`. This does not impact downstream projects (e.g. parquet-cpp is unaffected) unless they provide implementations of these virtual interfaces.

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #1363 from wesm/ARROW-1850 and squashes the following commits:

af5a3488 [Wes McKinney] Update glib, arrow-gpu for API changes
5d5cf2d0 [Wes McKinney] Use void* / const void* for buffers in file APIs