Title: #806: sudo: always use server highest usn for smart refresh
Because the search result contains only sudo object (or nothing), it is quite possible
that `srv_opts->max_sudo_value <= newusn < srv_opts->last_usn` therefore
before the patch we would set `srv_opts->max_sudo_value` to a value that is lower than
`srv_opts->last_usn`. With the patch we always use the highest known usn value and it
will get updated with each refresh.
See the full comment at https://github.com/SSSD/sssd/pull/806#issuecomment-488623909