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ARROW-1327: [Python] Always release GIL before calling check_status in Cython

This should prevent deadlock in some multithreaded or subinterpreter contexts. We can be more mindful of this in the future

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

Closes #945 from wesm/ARROW-1327 and squashes the following commits:

d690c5b3 [Wes McKinney] Fix some GIL acquisitions
870e5222 [Wes McKinney] Always release GIL before calling check_status in Cython