Replace lateral references to removed rels in subqueries

Replace lateral references to removed rels in subqueries

This commit introduces a new field 'sublevels_up' in ReplaceVarnoContext,
and enhances replace_varno_walker() to:
  1) recurse into subselects with sublevels_up increased, and
  2) perform the replacement only when varlevelsup is equal to sublevels_up.

This commit also fixes some outdated comments.  And besides adding relevant
test cases, it makes some unification over existing SJE test cases.

Discussion: https://postgr.es/m/CAMbWs4-%3DPO6Mm9gNnySbx0VHyXjgnnYYwbN9dth%3DTLQweZ-M%2Bg%40mail.gmail.com
Author: Richard Guo
Reviewed-by: Andrei Lepikhov, Alexander Korotkov