We should be able to
- update an existing collection so that it switches from no defined max_ttl to having a max_ttl
- update an existing collection to change the value of its max_ttl
Under the covers this requires changes to
- VB::Manifest to compare the max_ttl field against any incoming Manifest so we can trigger the change
- A new system-event which represents a change to a collection (which will be DCP replicated).