Eng 6436 distinct reroute

This is the fast, simple but not the best efficient solution for DISTINCT with GROUP BY cases. We will create tickets to optimize it later.
This could be refactored so that you always assert about the SendPlanNode and you always assert equality between apn1 and apn2 but based on pushdown you decide differently on how to set apn1 and apn2 -- if you have the assert, you can skip comparing the SendPlanNode in both cases, so apn1 would always the child but apn2 would be either the child or the grandchild.