Workaround for locally-handled sessions breaking on PHP 5.3 with APC enabled.
Big thanks to the folks at http://pecl.php.net/bugs/bug.php?id=16745 for the secret juju!
Classes were being torn down before session save handlers got called at the end of the request, which exploded with complaints about being unable to find various classes.
Registering a shutdown function lets us explicitly close out the session before everything gets torn down.