rename mail_user should update spamfilter_users

When changing the email address of a mail_user, the spamfilter_users entry for the old email remains, and a new spamfilter_users entry for the new email is created - that abandons all spamfilter_wblist entries formerly added for this mail_user, as they are still linked to the old spamfilter_users entry.

We should also fire spamfilter_users_update and spamfilter_wblist_update events, so configuration files (eg. rspamd) are rewritten.