WL#1165: Authorization features

Rewrite linear search of user in acl_getroot with hash. We can't do the same
for hosts, because we support host masks. -- needs to be done

Kristofer Petterson comments:
"Moving acl_users to a hash seems like a good idea. However, I'm not
aware of any report actually stating that this is a bottle neck today.
LOCK_grant is known to show up every so often in Dmitris reports though
so maybe it should be investigated more."

Georgi Kodinov comments:
"Since now find_mpvio_user can be done twice (if there's a proxy user involved)
I'd say we have twice the incentive to do this."

[ Four other suggestions for this worklog are cancelled.
See progress note for 2010-12-14. ]