sdap: update last_usn on reconnect
    
    If we are reconnecting to the same server it makes sense the keep the
    stored maximum USN values for the different object classes. But since
    the highest USN is already read from the rootDSE during the reconnect it
    make sense to keep this value to be able to update the maximum USN
    values even of no new object where found during the related searches.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>