Split out success probability calculation to allow for changes

Split out success probability calculation to allow for changes

Our "what is the success probability of paying over a channel with
the given liquidity bounds" calculation is reused in a few places,
and is a key assumption across our main score calculation and the
historical bucket score calculations.

Here we break it out into a function to make it easier to
experiment with different success probability calculations.

Note that this drops the numerator +1 in the liquidity scorer,
which was added to compensate for the divisor + 1 (which exists to
avoid divide-by-zero), making the new math slightly less correct
but not by any material amount.