title: How to get expiration time from cache
body: <p>With <em>django.core.cache.backends.locmem.LocMemCache</em> this worked:</p>
  
  <pre><code>key = cache.make_key('foo')
  cache.validate_key(key) 
  t = cache._expire_info.get(key)
  </code></pre>
  
  <p>But it breaks with <em>django.core.cache.backends.memcached.MemcachedCache</em></p>
  
  <p>I get error: <strong>'MemcachedCache' object has no attribute '_expire_info'</strong></p>
  
  <p>Is there an universal or memcached-specific way to get expiration time for specified key?</p>