Cache non existing DB user

We always query the database backend. Even if we use a different one
(ldap for example). Now we do this everytime we try to get a user object
so caching that a user is not in the DB safes some queries on each
request then (at least 2 what I found).

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>