The message in the response stream changed. Rename the calls themselves,
to prevent older applications from getting decode errors. Especially
troublesome is the case where the request is executed (send payment),
but the application can't read the outcome (payment sent or not?)
This commit fixes the inconsistency between the payment state as
reported by routerrpc.SendPayment/routerrpc.TrackPayment and the main
rpc ListPayments call.
In addition to that, payment state changes are now sent out for every
state change. This opens the door to user interfaces giving more
feedback to the user about the payment process. This is especially
interesting for multi-part payments.
