Add an amount penalty to ProbabilisticScorer

Add an amount penalty to ProbabilisticScorer

The cost of large payments tends to be dominated by the channel fees. To
avoid this, add an amount penalty to ProbabilisticScorer with a user
configurable multiplier. The multiplier is applied for every 2^20th of
the amount weighted by the negative log10 of the channel's success
probability for the payment.