Include the batch sweep transaction hash to `SwapStatus`

[hieblmi]

trafficstars

looking for concept ack, will then remove the draft status

This PR adds the batch sweep transaction hash to the SwapStatus rpc message. Next to the htlc outpoint it provides on-chain information about a loop-out swap.

In order to show the latest published batch transaction that sweeps the htlc output of a loop-out swap we add an additional channel SweepTxHashChan to the sweepbatcher.SpendNotifier to signal to the swap server that a batch transaction was just published that contained the loop-out htlc output.