[C++][Compute] GroupBy: add parallelism to hash group by

[asfimport]

Implement parallel processing for hash group by. 
Make sure it works well (scales) for different cardinalities of groups.

Reporter: Michal Nowakiewicz / @michalursa

Related issues:

• [C++] Query engine umbrella issue (is a child of)

_{Note: This issue was originally created as ARROW-12726. Please see the migration documentation for further details.}

[asfimport]

Michal Nowakiewicz / @michalursa:
Parallelism is implemented in hash group by. What remains is to benchmark to see how well it behaves for a set of varying dimensions, fix performance issues, investigate improvements for different cardinalities of groups, implement the improvements.