Add (minimal) text about expiry of catalog zones

[wtoorop]

what shall happen when a catalog zone expires? 1) Catalog zone shall never expire 2) Member zones shall be expired 3) Member zones shall be purged with their metadata 4) .... ?

[Habbie]

An implementation may not be able to prevent expiry of a zone under normal XFR management, but such expiry should lead to the catalog zone being treated as invalid (which means nothing changes), not as empty.

[libor-peltan-cznic]

I agree. Btw, current behaviour of Knot DNS: when interpreted catalog zone expires, it expires as a normal zone (log message, stops answering queries to the catalog zone itself), but it does nothing with its member zones: they are still configured based on persistent catalogDB, with the reference to the expired (but still configured) catalog zone. Even after server re-boot.

I guess we can leave to the implementation, if the (debug) queries to interpreted catalog zone are working after its expiry or not.

I assume we need to clarify this somehow in our draft, and revise all mentions of "expir[a-z]*" there.

[wtoorop]

this is addressed in the merge of #38 and can be closed once version-06 is accepted/merged

[wtoorop]

Well this part has been accepted so closing already