Before this commit `fee_amount` and `amount_needed` were passed as independent
parameters. From the perspective of coin selection algorithms, they are always
used jointly for the same purpose, to create a coin selection with a total
effective value greater than it's summed values.
This commit removes the abstraction that the use of the two parameter
introduced by consolidating both into a single parameter, `target_amount`, who
carries their values added up.
e8df3d2d91