Refactor to reduce code duplication for function property checking.

As noted by Andres Freund, we'd accumulated quite a few similar functions
in clauses.c that examine all functions in an expression tree to see if
they satisfy some boolean test.  Reduce the duplication by inventing a
function check_functions_in_node() that applies a simple callback function
to each SQL function OID appearing in a given expression node.  This also
fixes some arguable oversights; for example, contain_mutable_functions()
did not check aggregate or window functions for mutability.  I doubt that
that represents a live bug at the moment, because we don't really consider
mutability for aggregates; but it might someday be one.

I chose to put check_functions_in_node() in nodeFuncs.c because it seemed
like other modules might wish to use it in future.  That in turn forced
moving set_opfuncid() et al into nodeFuncs.c, as the alternative was for
nodeFuncs.c to depend on optimizer/setrefs.c which didn't seem very clean.

In passing, teach contain_leaked_vars_walker() about a few more expression
node types it can safely look through, and improve the rather messy and
undercommented code in has_parallel_hazard_walker().

Discussion: <20160527185853.ziol2os2zskahl7v@alap3.anarazel.de>

src/include/optimizer/planmain.h

@@ -107,9 +107,6 @@ extern bool query_is_distinct_for(Query *query, List *colnos, List *opids);
  * prototypes for plan/setrefs.c
  */
 extern Plan *set_plan_references(PlannerInfo *root, Plan *plan);
-extern void fix_opfuncids(Node *node);
-extern void set_opfuncid(OpExpr *opexpr);
-extern void set_sa_opfuncid(ScalarArrayOpExpr *opexpr);
 extern void record_plan_function_dependency(PlannerInfo *root, Oid funcid);
 extern void extract_query_dependencies(Node *query,
 						   List **relationOids,