Add a delete_queue to the ctdb database context struct.

This list will be filled by the client using a new
delete control. The list will then be used to implement
a fast-path vacuuming that will traverse this list instead
of traversing the database.

(This used to be ctdb commit 9bbedf786b26bb074f668b31f29a9032af958673)

ctdb/include/ctdb_private.h

@@ -514,6 +514,7 @@ struct ctdb_db_context {
 	struct lockwait_handle *lockwait_active;
 	struct lockwait_handle *lockwait_overflow;
 	struct ctdb_persistent_state *persistent_state;
+	struct trbt_tree *delete_queue;
 };