Avoid uncommon CoinJoin output amounts

When large UTXOs are mixed, the mixed output amount was chosen as the
smallest preprogramed amount which would result in one or more mixed
outputs.  This had the unfortunate side effect of advertising seldom
used mixed output amounts to a CoinShuffle++ server which were
uncommon relative to amounts advertised by clients participating in
mixed ticket buying.

As a workaround, the current sdiff is queried, and a decision is made
whether to use an even smaller mixed amount which will allow for
better pairings.

wallet/mixing.go

@@ -56,6 +56,11 @@ type DialFunc func(ctx context.Context, network, addr string) (net.Conn, error)
 func (w *Wallet) MixOutput(ctx context.Context, dialTLS DialFunc, csppserver string, output *wire.OutPoint, changeAccount, mixAccount, mixBranch uint32) error {
 	op := errors.Opf("wallet.MixOutput(%v)", output)
 
+	sdiff, err := w.NextStakeDifficulty(ctx)
+	if err != nil {
+		return errors.E(op, err)
+	}
+
 	var updates []func(walletdb.ReadWriteTx) error
 
 	hold, err := w.holdUnlock()
@@ -92,6 +97,17 @@ func (w *Wallet) MixOutput(ctx context.Context, dialTLS DialFunc, csppserver str
 	var count int
 	var mixValue, remValue dcrutil.Amount
 	for i, v := range splitPoints {
+		// When the sdiff is more than four times this mixed output
+		// amount, there is a smaller common mixed amount with more
+		// pairing activity (due to CoinShuffle++ participation from
+		// ticket buyers).  Skipping this amount and moving to the next
+		// smallest common mixed amount will result in quicker pairings,
+		// or pairings occurring at all.  Unlike any downmixing of mixed
+		// ticketbuying change, this will result in four or more outputs
+		// when mixing larger UTXOs.
+		if i != len(splitPoints)-1 && 4*v >= sdiff {
+			continue
+		}
 		count = int(amount / v)
 		if count > 0 {
 			remValue = amount - dcrutil.Amount(count)*v