Use phalcon_fetch_static_property_ce and phalcon_update_static_property_ce

Pros:
  * phalcon_fetch_static_property_ce() does not need PHALCON_OBS_VAR()
  * phalcon_fetch_static_property_ce() and phalcon_update_static_property_ce() use zend_class_entry — this saves hash table lookup.

Plus:
  * use interned strings in \Phalcon\Tag