Automatically maintain leases

The premise of the plugin is that leases are maintained with a scarce resource (ZKAPs) and shares without leases can be garbage collected to reclaim another scarce resource (storage).

For this to work, something must actually maintain leases on shares which are meant to remain alive. There is an existing UI for this in the Tahoe-LAFS CLI, eg tahoe deep-check --add-lease. The user could keep their data alive by using this to manually renew leases.

It should be possible to provide a better experience by renewing leases automatically, though. This removes the need for the user to remember to perform the task and creates the possibility that the task can be performed in a way that leaks less information to observers (the user may fall into patterns that can be identified, automation may be able to avoid doing so).