+ :param ptr: pointer to initialized `memcached_st` struct
+ :param group_key: the `key` namespace
+ :param group_key_length: the length of `group_key` without any terminating zero
+ :param key: the key to touch
+ :param key_length: the length of `key` without any terminating zero
+ :param expiration: new expiration as a unix timestamp or as relative expiration time in seconds
+ :returns: `memcached_return_t` indicating success