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ARROW-1335: [C++] Add offset to PrimitiveArray::raw_values to make consistent with other raw_values

This is an API change, but fixes an existing inconsistency that was the source of several bugs that had gone unnoticed because they were only being tested with code having 0 offset (i.e. unsliced). We'll need a corresponding patch in parquet-cpp

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

Closes #949 from wesm/ARROW-1335 and squashes the following commits:

10431ebf [Wes McKinney] Use raw_values in more places
3c96eb4a [Wes McKinney] Add offset to PrimitiveArray::raw_values to make consistent with other raw_values functions