Document catalog zones member zone reset by change of unique label

Document catalog zones member zone reset by change of unique label

The DNS catalog zones draft version 5 document requires that catalog
zones consumers must reset the member zone's internal zone state when
its unique label changes (either within the same catalog zone or
during change of ownership performed using the "coo" property).

BIND already behaves like that, and, in fact, doesn't support keeping
the zone state during change of ownership even if the unique label
has been kept the same, because BIND always removes the member zone
and adds it back during unique label renaming or change of ownership.

Document the described behavior and add a log message to inform when
unique label renaming occurs.

Add a system test case with unique label renaming.