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ARROW-709: [C++] Restore type comparator for DecimalType

Needed for PARQUET-923.

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

Closes #439 from wesm/ARROW-709 and squashes the following commits:

55d23a3 [Wes McKinney] Restore type comparator for DecimalType