Re: Schema variables - new implementation for Postgres 15

Yep, in this constellation the implementation holds much better (in
terms of memory) in my create/let/drop testing.

I've marked the CF item as ready for committer, but a note for anyone
who would like to pick up it from here -- we're talking about first 5
patches here, up to the memory cleaning after DROP VARIABLE. It doesn't
mean the rest is somehow not worth it, but I believe it's a good first
step.