Revert "Always use in-memory cache on top of permanent cache storage system"

This reverts commit aa643527d85dbe874c51e21185df3c2c28bc5a42.

Sleeping on this ... don't think there is anything wrong with the
concept, but be ultra-conservative considering this is a point release
change.

Most backends shouldn't have an issue with multiple requests, if written
well.  Perms does (as of now) so keep this for that code.
Use new Horde_Cache syntax