Aspen - Improve Lease Setting Option

[emilbon99]

The current syntax for the lease option for kv.Set is error prone and needs to be improved. Instead of requiring the leaseholder to be passed as the first option, we should use an options style pattern instead that that does an interface assertion instead. This will make the code much more robust, will enable us to add additional options to aspen and pass options to the underlying key-value store.