[Poloniex] add CurrencyPair param to cancelOrder() raw method

Typically an exchange only requires an `orderId` parameter to cancel an order.
The Poloniex cancelOrder API method requires 2 parameters: `orderId` and `currencyPair`

The current `xchange-poloniex` implementation will make an extra call to getOpenOrders() to find the CurrencyPair of an order; the order is then cancelled by providing `orderId` + `currencyPair` to the Poloniex API.

I've added an override to Poloniex's cancelOrder raw method to avoid the extra getOpenOrders() API call (if the caller already knows the CurrencyPair).