We discussed introducing a "PlanContext" struct to the query system similar to agg's ExpressionContext struct. This would hold all data that needs to be shared by all parts of a single query plan, including the WorkingSet and an OperationContext pointer. This would allow us to alter the OperationContext pointer in a single place during getmores rather than in each stage.

Now just having PlanStage manage the OperationContext* in the base class rather than having the children stages manage their own.