[Dovecot] Userdb authentication cache

Committed to CVS HEAD already. I think this is small enough change that it won't break anything too badly for v1.0, but I'd like to get some testing first. Here's a patch against branch_1_0 CVS and latest nightly snapshot (looks like they haven't been updating for a while, started building new a new one now):

http://dovecot.org/patches/userdb-cache.diff

Please try and tell me if it breaks something. :)