Implement non-blocking `melt` that makes use of new `pending` state

[callebtc]

In #277 (see NUT-07 update) we implemented a new pending state that is returned as a response to POST /check. With this, it is possible to implement a non-blocking Wallet.melt() method that does not need to keep the connection open until the Lightning invoice is paid (or fails). Instead, we could fire it off and recheck periodically using POST /check whether the invoice is still pending, succeeded, or failed.