* exchanges/ftx: add support for ReduceOnly, ImmediateOrCancel and PostOnly orders
* exchanges/ftx: adapt tests to ReduceOnly/ImmediateOrCancel/PostOnly boolean flags
* exchanges/ftx: wrong json key of WsFillsDataStore.FillsData was causing json.Unmarshal() to yield an empty struct
* exchanges/ftx: update WsFills to latest API
* exchanges/ftx: add TestFTX_wsHandleData_wsFills
* exchanges/ftx: WsOrders.FilledSize was never parsed properly because of a key error (was spelled `filedSize` -- with one L); enrich data structure to support latest API fields
* exchanges/ftx: when order is closed, set status to closed, not canceled
* exchanges/ftx: compatibleOrderVars: compare floats for almost-equality
* order.Detail: add field: AverageExecutedPrice float64
* exchanges/ftx: FTX.wsHandleData() now populates all order.Detail fields properly on update/orders websocket message
* exchanges/ftx: implement TestFTX_wsHandleData_Details()
* exchanges/ftx: make golangci-lint happy
* exchanges/ftx: make golangci-lint happy
* exchanges/ftx/test: delete redundant commented code
* exchanges/ftx: move websocket tests from ftx_test.go to ftx_websocket_test.go
* exchanges/ftx: remove redundant line
* exchange/ftx: greatly simplify TestFTX_wsHandleData (thanks to @shazbert)
* exchanges/ftx: make golangci-lint/whitespace happy
* exchanges/ftx: requested review change: delete a redundant line
* exchanges/ftx/test: use predefined currency constants
