title: spymemcached next unique key from cache
body: <p>Is it possible to get auto generated unique key using spymemcached API?</p>
  
  <p>I would like to use something like this:</p>
  
  <pre><code>String key = memcachedClient.add("Some data");
  //key - auto generated next unused key 
  </code></pre>
  
  <p>So, if it's impossible can I use some kind of sequences to get next unused key?</p>
  
  <p>Regards, Evgeniy</p>