With a separate proto message, it becomes possible to also return the
pair data for a single pair. This prepares for the new mc probability
querying rpc.
Probability estimates are amount dependent. Previously we assumed an
amount, but that starts to make less sense when we make probability more
dependent on amounts in the future.
This commit modifies paymentLifecycle so that it not only feeds
failures into mission control, but successes as well.
This allows for more accurate probability estimates. Previously,
the success probability for a successful pair and a pair with
no history was equal. There was no force that pushed towards
previously successful routes.
Previously mission control tracked failures on a per node, per channel basis.
This commit changes this to tracking on the level of directed node pairs. The goal
of moving to this coarser-grained level is to reduce the number of required
payment attempts without compromising payment reliability.
