pub fn touch(&mut self, key: &K, ttl: Option<Timestamp>, now: Timestamp) -> bool {
        // update the timestamp and last-used field of a row without copying the
        // whole contents
        match self._get_full_entry(key, now) {
            Some(full_entry) => {
                self._touch(key, full_entry, ttl, now);
                true
            },
            _ => false
        }
    }