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ARROW-2004: [C++] Add shrink_to_fit parameter to BufferBuilder::Resize, add Reserve method

I also relaxed the requirement to pass `const uint8_t*` so that one can pass `const void*` when writing to a `BufferBuilder`. This will not affect any downstream users

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

Closes #1486 from wesm/ARROW-2004 and squashes the following commits:

2d6660a8 [Wes McKinney] Add shrink_to_fit parameter to BufferBuilder::Resize, add Reserve method, relax pointer type in Append