Move current coin selection algorithm to coinselection.{cpp,h}

Summary:
Moves the current coin selection algorithm out of SelectCoinsMinConf
and puts it in coinselection.{cpp,h}. The new function, KnapsackSolver,
instead of taking a vector of COutputs, will take a vector of CInputCoins
that is prepared by SelectCoinsMinConf.

This is a partial backport of Core PR10637 : https://github.com/bitcoin/bitcoin/pull/10637/commits/fb716f7b25927e377f73b904a88ab67facfe3e55

Depends on D3246 and D3250

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3289