sysdb: do not use objectClass for users and groups
    
    The majority of the object in the SSSD cache are users and groups. If
    there are many user and groups in the cache the index objects of the
    objectclass attributes 'user' and 'group' become  large because the
    must hold references to all objects of those object classes.
    
    As a result the management of these index objects becomes costly because
    they must be parsed and split apart quite often. Additionally they are
    mostly useless because user and groups are lookup up by more specific
    attributes in general.
    
    Only when enumerating all user or groups this kind of index might be
    useful.
    
    There are two way of removing this kind of index from the user and group
    objects. Either by removing objectClass from the list of indexes and add
    a new attribute to all other type of object we want and index for. Or by
    replacing objectClass with a different attribute for the user and group
    objects. After some testing I think the latter one is the more reliable
    one and implemented it in this patch.
    
    Related to https://pagure.io/SSSD/sssd/issue/3503
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>