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ARROW-1338: [Python] Do not close RecordBatchWriter on dealloc in case sink is no longer valid

Also add missing close() statements to test_mock_output_stream to fix invalid writes causing core dump on OS X.

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

Closes #952 from wesm/ARROW-1338 and squashes the following commits:

88e8cefe [Wes McKinney] Do not close RecordBatchWriter on dealloc in case sink is no longer valid. Add missing close() statements to test_mock_output_stream