Add a dust-limit to `CoinSelect` and let the fee estimation consider if a change output is needed or not, assuring that if the change is below the dust-limit it will go towards the fee instead.
Without this change the high-fee logic is never tested as it is instead caught by the dust-output logic. This change uses a higher fee rate to ensure an output value above the dust limit, while still spending 20% on fees.
In this commit, we make an incremental change to move the existing coin
selection code into a new chanfunding package. In later commits, this
package will grow to serve all the lower level channel funding needs in
the daemon.
