Expired vouchers should be deleted in a cron

[luandro]

A cron should check for vouchers that have expired and delete them in order to save memory.

[patogit]

Having a record of used vouchers can be useful for analyzing network usage patterns and adjusting network administration accordingly. Maybe automatic deletion can be an option?

[luandro]

That fits well with the idea of #28, where real persistence can be achieved by using an external USB drive.