UnknownRecordData should not be used to copy records from one message to another

[Philip-NLnetLabs]

UnknownRecordData is not safe when copying records from one message to another. The reason is that some record types may contain compressed names. When these are copied as UnknownRecordData, the compression pointer may point to something different in the new message.

The solution is to create a new type that know about all record types that contain names that may be compressed and is generic for all other types.