Avoid context cancellation race

In the event that a purchase tickets request context from the ticket
autobuyer is cancelled due to the changing sdiff window, addresses and
outpoints must still be watched.  Not doing so eventually leads to an
invalid UTXO set as relevant transactions become missed.

There is a small chance that this change may cause a hang during
process shutdown.  A better solution for this is included in a
comment.

Author: jrick

wallet/createtx.go

@@ -1378,9 +1378,11 @@ func (w *Wallet) purchaseTickets(ctx context.Context, op errors.Op,
 	// relevant transactions
 	var watchOutPoints []wire.OutPoint
 	defer func() {
-		if ctx.Err() != nil {
-			return
-		}
+		// Cancellation of the request context should not prevent the
+		// watching of addresses and outpoints that need to be watched.
+		// A better solution would be to watch for the data first,
+		// before publishing transactions.
+		ctx := context.TODO()
 		_, err := w.watchHDAddrs(ctx, false, n)
 		if err != nil {
 			log.Errorf("Failed to watch for future addresses after ticket "+