Support coinjoin transactions with no change output

[raedah]

As mentioned in pull requests #407 and #413, the current code is always assigning a change output, even in cases where there is no change, so the change output has a value of zero which creates an invalid transaction. #413 patches it so that it fails early and with an error msg. I am opening this issue to track fixing this case for when there is no change and the transaction can go through without a change output needed.

#407 (comment)
Quoting belcher here: "coinjoin sending with no change address (i.e. sweeping) has never really been supported. It's not clear where the maker fees would go. Although it might be useful in the rare occasion when the maker gets an order for exactly the right coinjoin amount."

[raedah]

This will be useful for allowing yield generators to make offers which clear out mix depth dust, as well as other offers which are intended to match utxo size with no change.