Improve YSQL query performance

[rkarthik007]

Jira Link: DB-2315
While this is a large bucket of work, the table below lists some of the key items that are necessary.

Status	Feature	GitHub Issue
✅	Index cost estimates should account for index and scan type	#4494
⬜️	Improve JOIN planning and execution	#4519
⬜️	Cost based query optimization using table statistics	#3103
⬜️	Improve scan performance	#3520

[saurik]

(btw all of those unchecked issues are now closed)