These must provide for degrees of freedom adjustment when yielding result

Reporter: Wes McKinney / @wesm

Related issues:

• [C++] C++ array kernels framework and execution buildout (umbrella issue) (is a child of)
• [C++][Compute] Implement stdev aggregate kernel (duplicates)
• [C++] Parallelize execution of ScalarAggregateFunction (relates to)
• [C++] Abstract aggregation kernel API (is related to)

_{Note: This issue was originally created as ARROW-3122. Please see the migration documentation for further details.}