Use session_status when available to better detect if the session exists

In php 5.4.0 the [session_status()](http://nl1.php.net/manual/en/function.session-status.php) function was implemented which allows to determine if the session was already started without weird checks.

This patch makes `SessionManager::sessionExists` use this function when available and falls back to the old behaviour on php<5.4
I don't think so. Either the function exists or it doesn't. If zf drops php 5.3 support we don't need the fallback.