This prepares for decoupling the result interpretation of a single
payment attempt from the information stored in mission control memory
on the history of a node pair. A planned follow-up where we store both
the last success and last failure requires this decoupling.
This commit changes mission control to partially base the estimated
probability for untried connections on historical results obtained in
previous payment attempts. This incentivizes routing nodes to keep all
of their channels in good shape.
