Don't use Zend_Cache in manifests

The overhead of Zend_Cache in manifests is too high - we don't
need LRU or tags, just somewhere to dump a bunch of data that
persists

You can replace the class used by defining SS_MANIFESTCACHE
to be a class that implements the ManifestCache interface
(we can't use the Config system to set this, as it isn't
initialised yet).