ZF-6926: use SplObjectStorage class within Zend_Pdf_ElementFactory


ElementFactory uses several object collections, which are really large for large enough PDF documents. All of them are organized as arrays with a string keys which are generated by objects. Each key generation needs string processing.


Only $_removedObjects is transformed to SplObjectStorage.

$_modifiedObjects entries have to be sorted by keys while PDF dump operation.

$_registeredObjects entries have to be referable by string reference (to check if it's already parsed)