Deprecate eZExpiryHandler::registerShutdownFunction() and explicitly call eZExpiryHandler::store() instead

Because the "working directory of the script can change inside the shutdown function under some web servers, e.g. Apache." (from register_shutdown_function() PHP.net documentation), we can end up with eZExpiryHandler::store() writing to a different path than the expected one.

Replace this with an explicit call to eZExpiryHandler::store(), and deprecate eZExpiryHandler::registerShutdownFunction(), eZExpiryHandler::shutdown() and eZExpiryHandler::$isShutdownFunctionRegistered.