Invalide Cache Identifier for many fe_groups

[achimfritz]

may be we should use md5() hash for usergroups:
$identifier = $prefix . '-root-' . $root . '-language-' . $language . '-groups-' . md5(implode('_', $groupIds)) . '-' . $visibility . '-' . substr(md5(json_encode($configuration)), 0, 10);
instead of
$identifier = $prefix . '-root-' . $root . '-language-' . $language . '-groups-' .implode('_', $groupIds) . '-' . $visibility . '-' . substr(md5(json_encode($configuration)), 0, 10);