Document & refactor logical optimizer

Recently we've hit few bugs (#2021, #2034, #2017) in the optimizer/master planner. The motivation for opening this issue that we suspect there might be more hidden bugs like the ones we've found.

The goal is to:

1. Simplify the related code
- Better variable names, smaller functions, more comments
2. Document what is being pushed down and what is pulled to the coordinator when the query involves
- Aggregates, DISTINCT, DISTINCT ON, HAVING, COUNT DISTINCT, LIMIT - OFFSET - ORDER BY, Window functions, Group by on distribution key / or not, subqueries (mostly pushdownable ones)
3. Improve the optimizer
- Some more refactoring based on the previous items
- Apply missing optimizations if any
- Fix any issues found