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ARROW-1493: [C++] Flush stream in PrettyPrint functions

I'm not sure how to test this.

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

Closes #1066 from wesm/ARROW-1493 and squashes the following commits:

ffbb9af8 [Wes McKinney] Flush stream in PrettyPrint functions