dm_btree_lookup_next_single
r = dm_btree_lookup_next_single(info, value64(n, i), key, rkey, value_le);
r = dm_btree_lookup_next_single(info, value64(n, i), key, rkey, value_le);
r = dm_btree_lookup_next_single(info, root, keys[level], rkey, value_le);