Issue#1643: Coinselection prunes extraneous inputs from ApproximateBestSubset

Improvement for [Issue#1643](https://github.com/bitcoin/bitcoin/issues/1643):
A further pass over the available inputs has been added to ApproximateBestSubset after a candidate set has been found. It will prune any extraneous inputs in the selected subset, in order to decrease the number of inputs and the resulting change.