Fix EZP-23037: Subtree criterion in Legacy Storage Content search does not scale

This PR resolves issue https://jira.ez.no/browse/EZP-23037

As described in JIRA, in order to avoid the change of behaviour this introduces separate Subtree criterion to be used by permission system. `ezcontentobject_tree` is always joined to be used for Subtree limitation conditions.
No phpdoc ? :-)