Fix assorted confusion between Oid and int32.

In passing, also make some debugging elog's in pgstat.c a bit more
consistently worded.

Back-patch as far as applicable (9.3 or 9.4; none of these mistakes are
really old).

Mark Dilger identified and patched the type violations; the message
rewordings are mine.

src/backend/access/heap/tuptoaster.c

@@ -2155,7 +2155,7 @@ toast_open_indexes(Relation toastrel,
 	 * wrong if there is nothing.
 	 */
 	if (!found)
-		elog(ERROR, "no valid index found for toast relation with Oid %d",
+		elog(ERROR, "no valid index found for toast relation with Oid %u",
 			 RelationGetRelid(toastrel));
 
 	return res;