Ballista refactor QueryStageExec in preparation for map-side shuffle

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
This issue is to track some refactoring work in preparation for implementing map-side shuffle.

Describe the solution you'd like
We have Ballista executor code that executes a partition and streams results to disk. I would like to move this code to QueryStageExec instead and structure it so that we can implement map-side shuffle.

Describe alternatives you've considered
None

Additional context
None