Deny writes to slots without an active lease

Storage usage is authorized in two ways. First, by requiring ZKAPs to add leases to shares and by eventually garbage collecting shares sometime after all their leases expire. Second, by denying write access to mutable shares when they have no unexpired leases. Read access is never denied to support using the grid itself to coordinate between devices (for example, to exchange ZKAPs or vouchers). This has the benefit of providing a more graceful exit from the service for users who decide to take their data elsewhere.

This ticket is about the second part: denying write access to mutable shares with expired leases.