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ARROW-1488: [C++] Implement ArrayBuilder::Finish in terms of FinishInternal based on ArrayData

I don't have strong feelings about the function names here, but this was backwards compatible and will prevent extra box-then-unbox steps in analytics code that utilizes builders for accumulating outputs

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

Closes #1191 from wesm/ARROW-1488 and squashes the following commits:

0c23ff53 [Wes McKinney] Rename Finish with ArrayData to FinishInternal
5313cab2 [Wes McKinney] Initial hack at refactoring Finish methods to return ArrayData