Remove stage delegation in favor of planning-time stage assignation

[gabotechs]

In preparation of the upcoming work, removes the stage delegation mechanism in favor of something simpler: assign the StageContexts to the ArrowFlightReadExec nodes as an extra planning step instead of dynamically during the query.

This PR ships several simplifications:

• Removes the stage delegation mechanism in favor of leaving an empty slot in the ArrowFlightReadExec that an extra assing_stages planning task will fill. All the StageContexts and the node urls are now known at planning-time.
• Removes the delegate-actor terminology in favor of just tasks
• Updates the physical plan assertions in the tests to also account for the new information known during planning time.

[robtandy]

This looks like the right step before the ExecutionStage work comes in a subsequent PR