Test stream cancellation of the TrackPayments rpc call. In order to achieve
this, ControlTowerSubscriber is converted to an interface, to avoid trying to
close a null channel when closing the subscription. By returning a mock
implementation of the ControlTowerSubscriber in the test that problem is
avoided.
Add a method 'SubscribeAllPayments' to the control tower, in order to be able to
subscribe to any payment, rather than subscribing to a specific payment hash.
Jesse de Wit