MultiGet() needs to mutate &$cids properly

cache_get_multiple() is supposed to take $cids by reference and mutate the array to remove found keys. We're not doing this at the moment, meaning a lot of code (notably all of Ctools) things we're missing the cache all the time.

https://api.drupal.org/api/drupal/includes%21cache.inc/function/cache_ge...

I solved this by adding an array_diff() as seen in the default DrupalDatabaseCache implementation, adding it to LCache's getMultiple() method:

  public function getMultiple(&$cids) {
    if (empty($cids)) return;
    $cache = array();
    foreach ($cids as $cid) {
      $c = $this->get($cid);
      if (!empty($c)) {
        $cache[$cid] = $c;
      }
    }
    $cids = array_diff($cids, array_keys($cache));
    return $cache;
  }