Keven Li

Deribit Doc missed a "label" field on user.trade.xxx event data. The "label" contains corresponding `label` parameter of the buy/sell request. Since user.trades.xxx event is earlier than user.orders.xxx, handling by `label`...

fix GetUserTradesByOrder returning wrong type, json unmashal error.

Implement CancelByLabel see: https://docs.deribit.com/#private-cancel_by_label together with a new test TestClient_CancelByLabel