Re-examine disposal approach of the inner PlanExecutor of DocumentSourceCursor aggregation stage. Currently, for standard cursors, the inner PlanExecutor is disposed and destroyed on reaching an EOF. For tailable cursors, it is kept alive until higher level PlanExecutor is disposed. Keeping the PlanExecutor alive in all usage scenarios (whether disposed or not) is one of the alternatives to consider that could simplify the DocumentSourceCursor logic.