wallet: ensure VSP payment doesn't create unmixable change

[matthawkins90]

Summary

Buying tickets through a VSP had the possibility of creating outputs that were larger than dust, but not larger than the smallestMixChange. This change is toxic (since it is unmixable), and it also is linked to the ticket purchase. In practice, it's just more annoying than anything, because the wallet's unmixed balance will never reach zero.

Since ReserveOutputsForAmount is a public api, I created reserveOutputsForAmount to do the same thing, but also add minChange as an argument.

Then, CreateVspPayment calls that new function, and requests inputs totaling at least (fee + minMixableChange + estTxFee).

Edge Case Note

If this function uses a single input, the math works fine. However, if multiple inputs are used and they total the EXACT target (zero buffer), the actual txFee for two inputs will be higher than the estimated fee rate, and so we'll still end up with unmixable change.

I considered adding a log somewhere like:

if !txrules.IsDustAmount(dcrutil.Amount(change), txsizes.P2PKHPkScriptSize, feeRate) {
	minMixChange := smallestMixChange(feeRate)
	if dcrutil.Amount(change) < minMixChange {
		log.Warnf("VSP payment change %v is less than minimum mixable %v",
		dcrutil.Amount(change), minMixChange)
	}
	changeOut.Value = change
	tx.TxOut = append(tx.TxOut, changeOut)
	txauthor.RandomizeOutputPosition(tx.TxOut, len(tx.TxOut)-1)
}

but I figured that this might be an extreme edge case, and might be completely unnecessary?