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.
The former way to compute and create change was inside `create_tx`, just after
performing coin selection.
It blocked the opportunity to have an "ensemble" algorithm to decide between
multiple coin selection algorithms based on a metric, like Waste.
Now, change isn't created inside `coin_select` but the change amount and the
possibility to create change is decided inside the `coin_select` method. In
this way, change is associated with the coin selection algorithm that generated
it, and a method to decide between them can be implemented.
Cesar Alvarez Vallero