If a node contains a channel, but doesn't have a corresponding edge in
the graph database, updating the channel policy would fail. In this
commit the edge is recreated if the channel exists. This ensures a node
can recover from a missing edge in the graph database by calling
updatechanpolicy.
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