Prevent downloading shares with no active lease
As described in https://docs.google.com/document/d/1Bs13KTIr7UFalZlZM5D5gfh6NbZVRsu2JeAzCmZHIDI/edit#heading=h.jjky0uaaqde0 we may want a way to deny read access to data the storage-time of which has not been authorized - but without going all the way to outright deleting that data such that not even we can recover it.
This is useful as an intermediate step towards having garbage collection at all and it may be a nice operational mode to use in combination with garbage collection so that users are warned that their data is at risk before it is lost.