Anti-Spam relatively slow on login (rework antispam_get_moderator_types)

Hello,

While benchmarking hook_user_load I have noticed that the antispam module is quite slow. The whole antispam_get_moderator_types logic is perfectly correct, but somehow inefficient, given that usually a site has many normal users and few moderators.

My suggestion would be to have a helper function that builds up arrays of moderators and keeps them in a cached variable.

Does this make sense? How do we figuere when to flush this variable?