subquery merge re-write

There is a number of very banal query rewrite optimizations that we are not doing, paying an ugly performance price. Main examples are things like
failure to flatten queries e.g. select count(c) from (select * from tbl). This epic is designed to hold all tickets related to it.
!!! Before we implement this let's measure the impact b/c there might be no performance gain, e.g. select count(c) from (select * from tbl) vs select count(c) from tbl