Adjust definition of cheapest_total_path to work better with LATERAL.

In the initial cut at LATERAL, I kept the rule that cheapest_total_path
was always unparameterized, which meant it had to be NULL if the relation
has no unparameterized paths.  It turns out to work much more nicely if
we always have *some* path nominated as cheapest-total for each relation.
In particular, let's still say it's the cheapest unparameterized path if
there is one; if not, take the cheapest-total-cost path among those of
the minimum available parameterization.  (The first rule is actually
a special case of the second.)

This allows reversion of some temporary lobotomizations I'd put in place.
In particular, the planner can now consider hash and merge joins for
joins below a parameter-supplying nestloop, even if there aren't any
unparameterized paths available.  This should bring planning of
LATERAL-containing queries to the same level as queries not using that
feature.

Along the way, simplify management of parameterized paths in add_path()
and friends.  In the original coding for parameterized paths in 9.2,
I tried to minimize the logic changes in add_path(), so it just treated
parameterization as yet another dimension of comparison for paths.
We later made it ignore pathkeys (sort ordering) of parameterized paths,
on the grounds that ordering isn't a useful property for the path on the
inside of a nestloop, so we might as well get rid of useless parameterized
paths as quickly as possible.  But we didn't take that reasoning as far as
we should have.  Startup cost isn't a useful property inside a nestloop
either, so add_path() ought to discount startup cost of parameterized paths
as well.  Having done that, the secondary sorting I'd implemented (in
add_parameterized_path) is no longer needed --- any parameterized path that
survives add_path() at all is worth considering at higher levels.  So this
should be a bit faster as well as simpler.

src/backend/optimizer/path/allpaths.c

@@ -722,7 +722,7 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 		 * the unparameterized Append path we are constructing for the parent.
 		 * If not, there's no workable unparameterized path.
 		 */
-		if (childrel->cheapest_total_path)
+		if (childrel->cheapest_total_path->param_info == NULL)
 			subpaths = accumulate_append_subpath(subpaths,
 											 childrel->cheapest_total_path);
 		else
@@ -932,7 +932,6 @@ generate_mergeappend_paths(PlannerInfo *root, RelOptInfo *rel,
 				cheapest_startup = cheapest_total =
 					childrel->cheapest_total_path;
 				/* Assert we do have an unparameterized path for this child */
-				Assert(cheapest_total != NULL);
 				Assert(cheapest_total->param_info == NULL);
 			}