title: Symfony2 not respecting my session lifetime setting
body: <p>I have my app set, in <code>app/config/config.yml</code>, to have a session lifetime of one year:</p>
  
  <pre><code>framework:
      session:
          default_locale: %locale%
          lifetime:       31536000
          auto_start:     true
  </code></pre>
  
  <p>However, my users are getting booted after about an <em>hour</em> of inactivity, which is of course less than a year. Any ideas on what the problem could be?</p>