[NFR] Use Zend iterator functions for APCIterator instead of userspace ones

APCIterator, being native Zend class, provides faster access to its Iterator interface via zend_object_iterator.

zend_object_iterator methods are faster because they are called as C functions vs call via userspace.

On the other hand, hardly is `Phalcon\Cache\Backend\Apc::queryKeys()` called so often to see the difference.