schema: Introduce schema cache format
    
    Information about schema cache format is stored in every cache item.
    When schema cache format changes in incompatible way format will be
    increased. When format stored in cache doesn't match currently used
    format the entry in cache is ignored.
    
    https://fedorahosted.org/freeipa/ticket/6048
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>