Use rewrite instead of local Session model for form key handling

As I described in https://github.com/nexcess/magento-turpentine/issues/491 the session model can simply be rewritten in config.xml rather than use a copy in the local codepool to implement the form key placeholder handling, since ```Mage_Core_Model_Session``` is not in fact the base class of all of the other session classes, but ```Mage_Core_Model_Session_Abstract``` is. I have verified this back to Magento 1.6.