Allow non-expiring views
This release introduces the notion of 'non-expiring' views by setting a ttl
property of -1
in the view's spec.
The purpose of this enhancement is to allow generation of views that will never change directly, and thus have no need for an impact index (but also have no need to expire).
This change does have an effect on existing views with ttl properties: if the RDF type of the view's resource changes, it will regenerate the view's graph, regardless of whether or not the TTL has been met.