Score in-flight amounts as amounts, not a capacity reduction

When we started considering the in-flight amounts when scoring, we
took the approach of considering the in-flight amount as an
effective reduction in the channel's total capacity. When we were
scoring using a flat success probability PDF, that was fine,
however in the next commit we'll move to a highly nonlinear one,
which makes this a pretty confusing heuristic.

Here, instead, we move to considering the in-flight amount as
simply an extension of the amount we're trying to send over the
channel, which is equivalent for the flat success probability PDF,
but makes much more sense in a nonlinear world.

Author: TheBlueMatt