Modifies the payment session to launch additional pathfinding attempts
for lower amounts. If a single shot payment isn't possible, the goal is
to try to complete the payment using multiple htlcs. In previous
commits, the payment lifecycle has been prepared to deal with
partial-amount routes returned from the payment session. It will query
for additional shards if needed.
Additionally a new rpc payment parameter is added that controls the
maximum number of shards that will be used for the payment.
Adds an integrated routing test of probability extrapolation for untried
channels. The larger part of this commit is mock code to simulate the
Lightning Network.
The difference between this test and the existing pathfinding tests, is that
this test focuses on the feedback loop from result interpretation via
mission control updates and probability estimation back to pathfinding.
Improvements like probability extrapolation were previously only
validated by reasoning, while this setup makes it possible to assert the
improvement in a test and guard it for the future.
