Measure the number of all-visible pages for use in index-only scan costing.

Add a column pg_class.relallvisible to remember the number of pages that
were all-visible according to the visibility map as of the last VACUUM
(or ANALYZE, or some other operations that update pg_class.relpages).
Use relallvisible/relpages, instead of an arbitrary constant, to estimate
how many heap page fetches can be avoided during an index-only scan.

This is pretty primitive and will no doubt see refinements once we've
acquired more field experience with the index-only scan mechanism, but
it's way better than using a constant.

Note: I had to adjust an underspecified query in the window.sql regression
test, because it was changing answers when the plan changed to use an
index-only scan.  Some of the adjacent tests perhaps should be adjusted
as well, but I didn't do that here.

src/backend/nodes/outfuncs.c

@@ -1743,6 +1743,7 @@ _outRelOptInfo(StringInfo str, RelOptInfo *node)
 	WRITE_NODE_FIELD(indexlist);
 	WRITE_UINT_FIELD(pages);
 	WRITE_FLOAT_FIELD(tuples, "%.0f");
+	WRITE_FLOAT_FIELD(allvisfrac, "%.6f");
 	WRITE_NODE_FIELD(subplan);
 	WRITE_NODE_FIELD(subroot);
 	WRITE_NODE_FIELD(baserestrictinfo);