Rethink treatment of "postponed" quals in deconstruct_jointree().

Rethink treatment of "postponed" quals in deconstruct_jointree().

After pulling up LATERAL subqueries, we may have qual clauses that
refer to relations outside their syntactic scope.  Before doing any
such pullup, prepjointree.c checks to make sure that it wouldn't
create a semantically-invalid situation; but we leave it to
deconstruct_jointree() to actually move these quals up the join
tree to a place where they can be evaluated.  In commit 2489d76c4,
I (tgl) refactored deconstruct_jointree() in a way that caused
assertion failures while moving such quals, because the new logic
failed to distinguish "this jointree node is a parent of the source
one" from "this jointree node is processed after the source
one in depth-first order".

Fix this, and at the same time reduce the overhead a bit, by
getting rid of the common PostponedQual list and instead making each
JoinTreeItem contain a list of quals that needed to be postponed to
its level.  We can help distribute_qual_to_rels find the appropriate
JoinTreeItem efficiently by adding parent-item links to the
JoinTreeItem data structure.  This ends up being the same number
of relid subset checks as the original (pre-bug) logic, but less
list manipulation is required during multi-level postponements.

Richard Guo and Tom Lane, per bug #17768 from Robins Tharakan.

Discussion: https://postgr.es/m/17768-5ac8730ece54478f@postgresql.org