Incentivize cleanup with rebate

Similar to https://github.com/airswap/airswap-aips/issues/12

State growth in the session key registry can be curtailed by refunding a deposit when storage is cleared. Setting zeroes to nonzeroes should require a fixed deposit, to refunded when nonzeroes are set to zeros.

Additionally, provide a public method to clear out expired permissions to claim the rebate.

This would limit the extent to which state growth is possible. State growth can be limited further by restricting the expiry duration.