[Backport MB-47928 to 7.0.2] Need stat for load on index node calculation for Greedy planner testing

We need a stat for identifying load on any give Index node for deterministically finding the node for new index placement. Currently cbindexplan doesn't give any such stat and it involves calculation with multiple stats. It would be easier if we have the stat that Greedy/SA planner use to determine the exact placement of new index. 

After doing further testing, I found out that cbindexplan also doesn't give correct size estimation with deferred index. Due to this incorrect value expected results don't match with actual implementation.