always keep spamfilter_users entry
Currently when a mail_user is set to inherit the spamfilter policy from their domain, a spamfilter_user entry for that user is deleted. However, in amavisd-new, the wblist (spamfilter_wblist) lookups tie to the users (spamfilter_users) via an id, so if there is no spamfilter_users entry for a user/domain, that users/domain cannot have any spamfilter whitelist/blacklist addrs.
So we need to always keep the spamfilter_users entry for all mail_users, and track "inherit domain spamfilter policy" in a different manner; perhaps by setting spamfilter_users.policy_id to 0. Amavis' $sql_select_policy will need adjusted accordingly, and all places that handle spamfilter_users should be reviewed and modified if needed.
A cleanup run in the installer for abandoned wblist entries might also be good (fire a spamfilter_wblist_delete for each).