freshnessWarningThreshold config option

[Cantor, Scott]

> I don’t dispute that expirationWarningThreshold is a necessary config
> option. What I think is needed, however, is a complementary
> freshnessWarningThreshold config option that keys off @creationInstant
> (if present):

I would not be in favor of adding even more complexity at this point, especially when all of this is meant to be largely historical in favor of MDQ anyway.

> Similar to the RequiredValidUntil metadata filter, a filter that
> requires the @creationInstant attribute would be useful. Moreover, the
> filter should enforce the fact that consecutively published metadata
> documents must have @creationInstant values that are monotonically
> increasing. If a document fails this check, a consumer may conclude
> that the document has been replayed, intentionally or otherwise.

There's nowhere to store that state unless we put it in a file, and I think adding more moving parts and more state that might get broken and require purging would ultimately be a bad idea.

But if we wanted to deal with replay, I think the ID would be an obvious way to do that without relying on a schema extension.

-- Scott