NSS: Adjust netgroup setnetgrent cache lifetime if midpoint refresh is used
    
    This is a minor regression compared to the state of the code before we
    converted the responders to cache_req. The NSS responder keeps a has
    table of netgroup objects in memory for either the lifetime of the
    netgroup, or, in case midpoint refresh is used, up to the midpoint
    refresh time. The case with the midpoint refresh was removed in the
    cache_req enabled code, which means that even if the netgroup was
    updated in the cache with the background refresh task, the object was
    never read from cache, but always still returned from the in-memory
    enumeration hash.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3550
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>