arrow

ARROW-1185: [C++] Status class cleanup, warn_unused_result attribute and Clang warning fixes

This was tedious, but overdue. The Status class in Arrow as originally imported from Apache Kudu, which had been modified from standard use in Google projects. I simplified the implementation to bring it more in line with the Status implementation used in TensorFlow.

This also addresses ARROW-111 by providing an attribute to warn in Clang if a Status is ignored

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

Closes #814 from wesm/status-cleaning and squashes the following commits:

7b7e6517 [Wes McKinney] Bring Status implementation somewhat more in line with TensorFlow and other Google codebases, remove unused posix code. Add warn_unused_result attribute and fix clang warnings