SYSDB: Avoid optimisation with modifyTimestamp for users
    
    The usage of modifyTimestamp needn't be a reliable way
    for detecting of changes in user entry in LDAP.
    The authorisation need to rely current data from LDAP
    and therefore we will temporary disable optimisation with
    modifyTimestamp and we will rather rely on deep comparison
    of attributes. In he future, it might be changed and
    responders might control the optimization level.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3110
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>