current_user is run each time looking for current user

[evizitei]

In Quimbee we found that the current_user proc was being executed in excess of 20 times on each request to a tandem page. This wasn't a big deal if it was inexpensive, but when we introduced IP filtering, that proc introduced serious performance problems. We got around that in quimbee by using the session to set flags indicating what had already been checked, but we might consider having tandem cache the current_user information for a particular request so it only has to run the proc once in the request/response cycle.