DB_DNODE_LOCK
#define DB_DNODE_ENTER(_db) (zrl_add(&DB_DNODE_LOCK(_db)))
#define DB_DNODE_EXIT(_db) (zrl_remove(&DB_DNODE_LOCK(_db)))
#define DB_DNODE_HELD(_db) (!zrl_is_zero(&DB_DNODE_LOCK(_db)))