kraken futures: add support for market orders (#870)

Kraken Futures supports a `mkt` order type which is a market order at
best price with a 1% order protection.

exchanges/kraken/futures_types.go

@@ -13,6 +13,7 @@ var (
 		order.Stop:              "stp",
 		order.PostOnly:          "post",
 		order.TakeProfit:        "take_profit",
+		order.Market:            "mkt",
 	}
 
 	validSide = []string{"buy", "sell"}

exchanges/kraken/kraken_futures.go

@@ -116,7 +116,7 @@ func (k *Kraken) FuturesSendOrder(ctx context.Context, orderType order.Type, sym
 	size, limitPrice, stopPrice float64) (FuturesSendOrderData, error) {
 	var resp FuturesSendOrderData
 
-	if ioc {
+	if ioc && orderType != order.Market {
 		orderType = order.ImmediateOrCancel
 	}