Refactor Ballista to separate Flight logic from execution logic

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
This issue is to track a small refactor to clean up some tech debt.

Describe the solution you'd like
The Flight service should delegate to an executor struct for query execution. The flight service should handle converting the results into a flight stream.

Describe alternatives you've considered
None

Additional context
None