sweep: expand NotifyBroadcast to include an outpoint index

In this commit, we expand the `NotifyBroadcast` to include an outpoint
index. This is useful as it indicates the index of a given required tx
out input.

contractcourt/breach_arbitrator.go

@@ -752,7 +752,7 @@ justiceTxBroadcast:
 		}
 
 		return aux.NotifyBroadcast(
-			&bumpReq, finalTx.justiceTx, finalTx.fee,
+			&bumpReq, finalTx.justiceTx, finalTx.fee, nil,
 		)
 	})
 	if err != nil {