Invent "join domains" to replace the below_outer_join hack.

EquivalenceClasses are now understood as applying within a "join
domain", which is a set of inner-joined relations (possibly underneath
an outer join).  We no longer need to treat an EC from below an outer
join as a second-class citizen.

I have hopes of eventually being able to treat outer-join clauses via
EquivalenceClasses, by means of only applying deductions within the
EC's join domain.  There are still problems in the way of that, though,
so for now the reconsider_outer_join_clause logic is still here.

I haven't been able to get rid of RestrictInfo.is_pushed_down either,
but I wonder if that could be recast using JoinDomains.

I had to hack one test case in postgres_fdw.sql to make it still test
what it was meant to, because postgres_fdw is inconsistent about
how it deals with quals containing non-shippable expressions; see
https://postgr.es/m/1691374.1671659838@sss.pgh.pa.us.  That should
be improved, but I don't think it's within the scope of this patch
series.

Patch by me; thanks to Richard Guo for review.

Discussion: https://postgr.es/m/830269.1656693747@sss.pgh.pa.us

src/include/optimizer/paths.h

@@ -122,7 +122,7 @@ typedef bool (*ec_matches_callback_type) (PlannerInfo *root,
 
 extern bool process_equivalence(PlannerInfo *root,
 								RestrictInfo **p_restrictinfo,
-								bool below_outer_join);
+								JoinDomain *jdomain);
 extern Expr *canonicalize_ec_expression(Expr *expr,
 										Oid req_type, Oid req_collation);
 extern void reconsider_outer_join_clauses(PlannerInfo *root);

src/include/optimizer/planmain.h

@@ -84,7 +84,6 @@ extern RestrictInfo *process_implied_equality(PlannerInfo *root,
 											  Expr *item2,
 											  Relids qualscope,
 											  Index security_level,
-											  bool below_outer_join,
 											  bool both_const);
 extern RestrictInfo *build_implied_join_equality(PlannerInfo *root,
 												 Oid opno,