With the introduction of the slot-based execution (SBE) engine, the server will support two completely unrelated query execution engines. Luckily, PlanExecutor, the type which represents a query execution plan, was already an interface. However, some of the interface methods involved details of the pre-existing execution engine. We should narrow the interface so that it only contains methods which make sense for both the slot-based engine and the "classic" engine. For example, PlanExecutor::getRootStage() returns a pointer to a "classic" PlanStage, and therefore does not make sense in the context of SBE.