Don't clear btpo_cycleid during _bt_vacuum_one_page.

When "vacuuming" a single btree page by removing LP_DEAD tuples, we are not
actually within a vacuum operation, but rather in an ordinary insertion
process that could well be running concurrently with a vacuum.  So clearing
the cycleid is incorrect, and could cause the concurrent vacuum to miss
removing tuples that it needs to remove.  This is a longstanding bug
introduced by commit e6284649b9 of
2006-07-25.  I believe it explains Maxim Boguk's recent report of index
corruption, and probably some other previously unexplained reports.

In 9.0 and up this is a one-line fix; before that we need to introduce a
flag to tell _bt_delitems what to do.

src/backend/access/nbtree/nbtree.c

@@ -889,7 +889,7 @@ restart:
 		 */
 		if (ndeletable > 0)
 		{
-			_bt_delitems(rel, buf, deletable, ndeletable);
+			_bt_delitems(rel, buf, deletable, ndeletable, true);
 			stats->tuples_removed += ndeletable;
 			/* must recompute maxoff */
 			maxoff = PageGetMaxOffsetNumber(page);