Support index-only scans using the visibility map to avoid heap fetches.

When a btree index contains all columns required by the query, and the
visibility map shows that all tuples on a target heap page are
visible-to-all, we don't need to fetch that heap page.  This patch depends
on the previous patches that made the visibility map reliable.

There's a fair amount left to do here, notably trying to figure out a less
chintzy way of estimating the cost of an index-only scan, but the core
functionality seems ready to commit.

Robert Haas and Ibrar Ahmed, with some previous work by Heikki Linnakangas.

src/backend/optimizer/plan/planner.c

@@ -3297,7 +3297,7 @@ plan_cluster_use_sort(Oid tableOid, Oid indexOid)
 	/* Estimate the cost of index scan */
 	indexScanPath = create_index_path(root, indexInfo,
 									  NIL, NIL, NIL,
-									  ForwardScanDirection, NULL);
+									  ForwardScanDirection, false, NULL);
 
 	return (seqScanAndSortPath.total_cost < indexScanPath->path.total_cost);
 }