The previous commit moved preferences to using the Attributes cache;
however, this gains little because the cache is on the User object
derived from the CurrentUser, which is derived afresh each time that
RT->Config->Get() is called.

Call the ->Preferences method directly on the CurrentUser, instead of
the desrived User, so the cache is on the more durable CurrentUser
object.  The cache is cleared (via ->ClearAttributes) during the ->Load
call at the start of every request.  ->DeletePreferences is also
switched to use ->DeleteAttribute, which explicitly purges the cache;
otherwise, it may return stale data.

This better attribute caching ensures that even negative preferences
loads are cached for the duration of the request.  This, in turn,
eliminates a large number of repeated database queries (such as for the
username format preference) for users with no explicit preferences set.