Title: #806: sudo: always use server highest usn for smart refresh
I'm not sure what do you mean. From what I read in the code I see that:
1) `srv_opts->last_usn` is updated by reading `lastUSN` attribute from `rootDSE` when
new connection is established (sdap id op code or online check code)
2) It gets also updated when enumerating users/groups/services and sudo smart refresh if
any object found by the enumeration have `entryUSN` > `srv_opts->last_usn`.
So it does not necessarily corresponds to real value of `lastUSN` attribute but it
contains greatest USN value known to SSSD.
See the full comment at https://github.com/SSSD/sssd/pull/806#issuecomment-489600986