v1.5.6 fixed the wrong RelatedObjectDoesNotExist import and the catz
member-identifier creation, but two get_or_create() calls still used the
returned value directly instead of unpacking the (obj, created) tuple:
- increment_soa_serial(): `soa_serial_obj.value` on a tuple
- _create_soa_rdataset(): `.get_or_create(...).value` on a tuple
Both raised `'tuple' object has no attribute 'value'` at runtime — the
first on every zone save, the second on every catalog-zone (catz) query.
Unpack the tuple in both places.
