After ARROW-8972, we have a generic chunk-based executor for aggregate functions. It should be relatively straightforward now to parallelize the execution loop of the executor.

Reporter: Wes McKinney / @wesm

Related issues:

• [C++] Implement Hash Aggregation query execution node (is blocked by)
• [C++] C++ array kernels framework and execution buildout (umbrella issue) (is a child of)
• [C++] Mean kernel aggregate (is related to)
• [C++] Incremental Variance, Standard Deviation aggregators (is related to)
• [C++] Parallelize execution of arrow::compute::ScalarFunction (is related to)
• [C++] Abstract aggregation kernel API (is related to)

_{Note: This issue was originally created as ARROW-3120. Please see the migration documentation for further details.}