Zone diff support is incomplete/incorrect.

[ximon18]

When a zone is updated via the WritableZoneNode::update_rrset() method a diff of the effect can optionally be created.

No such support for creating diffs is implemented however when the "special" make_xxx() methods are used to update a zone, e.g. make_zone_cut() or make_cname().

Such updates will not appear in the created diff.

Additionally, changing the type of a node from "special" to "normal" by calling WritableZoneNode::update_rrset() on a node which was previously created using one of the make_xxx() methods will cause an incorrect diff to be recorded showing that the node changed from empty to having a "normal" RRset, while actually it went from having a "special" RRset to a "normal" RRset.