Let user control planning frequency/replanning policy

[eelstork]

• Whether replanning at every step, or duration based, or after attempting to fully execute a plan... implies tradeoffs that should be controlled by the designer.
• In some cases a plan can't execute correctly through re-planning. For an example, consider a "visit every square" goal. In this case knowledge of what's been covered or not is implicit to the solution - replanning would break that.