Add CheckTableNotInUse calls in DROP TABLE and DROP INDEX.

Recent releases had a check on rel->rd_refcnt in heap_drop_with_catalog,
but failed to cover the possibility of pending trigger events at DROP time.
(Before 8.4 we didn't even check the refcnt.)  When the trigger events were
eventually fired, you'd get "could not open relation with OID nnn" errors,
as in recent report from strk.  Better to throw a suitable error when the
DROP is attempted.

Also add a similar check in DROP INDEX.

Back-patch to all supported branches.

src/backend/catalog/heap.c

@@ -1236,6 +1236,13 @@ heap_drop_with_catalog(Oid relid)
 	 */
 	rel = relation_open(relid, AccessExclusiveLock);
 
+	/*
+	 * There can no longer be anyone *else* touching the relation, but we
+	 * might still have open queries or cursors, or pending trigger events,
+	 * in our own session.
+	 */
+	CheckTableNotInUse(rel, "DROP TABLE");
+
 	/*
 	 * Schedule unlinking of the relation's physical file at commit.
 	 */

src/backend/catalog/index.c

@@ -848,6 +848,12 @@ index_drop(Oid indexId)
 
 	userIndexRelation = index_open(indexId, AccessExclusiveLock);
 
+	/*
+	 * There can no longer be anyone *else* touching the index, but we
+	 * might still have open queries using it in our own session.
+	 */
+	CheckTableNotInUse(userIndexRelation, "DROP INDEX");
+
 	/*
 	 * Schedule physical removal of the file
 	 */