REP-819 Keystone V3 Filter

The mess that is Keystone v3! Comment on _everything_ so we can get this fixed up. =)
This was made package private to make testing easier. Being able to set the cached admin token reduces the amount of set-up for certain unit tests. Admittedly, that is only the case because of the linear, hierarchic execution path we take through the methods of this class. I would very much like to refactor a bunch of this code to simplify testing and remove oddities like this, but I'm not sure if now is the time for it. With all of that in mind, let me know how you think we should proceed.

I agree that it should be an Option, though. I'm not sure why I didn't use one to begin with. I'll make that change.