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ARROW-2454: [C++] Allow zero-array chunked arrays

This allows code to be more regular and less fragile.

Also fix the chunked array slicing logic.

Author: Antoine Pitrou <antoine@python.org>

Closes #1897 from pitrou/ARROW-2454-zero-length-chunked-arrays and squashes the following commits:

4ad2c6f <Antoine Pitrou> ARROW-2454:  Allow zero-array chunked arrays