User deletion requests.

    Those requests are logged in a different model to keep a trace of this action and the
    deletion is done in a CRON. Indeed, removing a user can be a heavy operation on
    large database (because of create_uid, write_uid on each model, which are not always
    indexed). This model just remove the users added in the deletion queue, remaining code
    must deal with other consideration (archiving, blacklist email...).
    