[dev] Permissions cache

[Chuck Hagenbuch]

Quoting Jan Schneider <jan at horde.org>:

> I don't think we should implement this as a custom driver, the cache
> should be available independant from the drivers. Use the Share and
> Group caching as a guideline.

I agree, with the caveat that I think this should be cached in  
Horde_Cache, not in the session. If we cache this data in the session  
then it can't be invalidated properly and it will balloon session  
size, hurting performance.

-chuck