This mode is useful for DDNS-type updates; it allows for additions and deletions to be done in any order (keeping track of them separately).
It does have some extra restrictions; SOA records must be the first deleted and the first added type, and may not occur later anymore (causes ValueError).

For now, I left this in python/isc/xfrin, even though it is not purely xfrin anymore. Should we move it to datasrc? or util?