dssync keytab: add support for keeping track of the up-to-date-ness vector.

dssync keytab: add support for keeping track of the up-to-date-ness vector.

The startup operation should get the old up-to-date-ness vector from the backend
and the finish operation should store the new vector to the backend after replication.

This adds the change of the signatures of the operations ot the dssync_ops struct
and the implementation for the keytab ops. The up-to-date-ness vector is stored
under the principal constructed as UTDV/$naming_context_dn@$dns_domain_name.

The vector is still uninterpreted in libnet_dssync_process().
This will be the next step...

This code is essentially by Metze.

Michael
(This used to be commit 01318fb27a1aa9e5fed0d4dd882a123ab568ac37)