ZoneUpdater doesn't handle "special" RR updates correctly.

[ximon18]

Currently an in-memory zone can be updated via update_rrset() passing a "special" RR type (a CNAME or zone cut (DS or NS)) and will blindly add these to the internal tree as "normal" nodes, but they should be added with special handling.

This is by design as there needs to be a way to faithfully construct the zone that is received via XFR to match the primary, with incorrect RRs as well.

The ZoneUpdater was primarily added to support the XFR case and so adds and removes records to/from the zone based on what is in the given ZoneUpdate as produced by XfrZoneInterpreter. However, while this will faithfully create both good and bad records in the tree, it will not create "special" records correctly thereby preventing query() (with the in-memory zone at least, behaviour of user supplied zone backing stores is implementation dependent) calls from responding correctly.

The ZoneUpdater should detect "special" cases and apply them via the appropriate make_xxx() methods on the WritableZoneNode interface instead of using update_rrset() for everything.