GateIO: Fix GetFuturesContractDetails for delivery futures and minor other fixes (#1766)

* GateIO: Fix GetFuturesContractDetails for Deliveries

Was returning the product of all the contracts, so 1444 instead of 38
contracts.

* GateIO: Fix GetOpenInterest returning asset.ErrNotEnabled

Using wrong error for pair not enabled

* GateIO: Rename GetSingleContract and GetSingleDeliveryContracts

Especially fixes GetSingleContract, which seems misleading to not say
Futures.
There's a load of `GetSingle*` here that should probably also be fixed,
but these two justified a dyno

* GateIO: Rename GateIOGetPersonalTradingHistory to GetMySpotTradingHistory

* GateIO: Rename GetMyPersonalTradingHistory to GetMyFuturesTradingHistory

* GateIO: Remove duplicate DeliveryTradingHistory

* GateIO: Rename Get*PersonalTradingHistory to GetMy*TradingHistory

* Linter: Disable shadow linting for err

It's been a year, and I'm still getting caught out by govet demanding I
don't shadow a var I was deliberately shadowing.
Made worse by an increase in clashes with stylecheck when they both want
opposite things on the same line.

* GateIO: Add missing Futures and tradinghistory fields

* GateIO: Improve WS Header parsing

This unifies handling for time_ms and time in response headers, since
options and delivery have only time, but spot has time_ms as well.
We use the better of the two results.

Also [improves performance 2x](https://gist.github.com/gbjk/7cacb63b9a256e745534bb05ca853c48)

* GateIO: Use time_ms WS fields where available

Removes the deprecated _time json fields and populates our Time fields
with the time_ms values

exchanges/subscription/template.go

@@ -129,7 +129,7 @@ func expandTemplate(e IExchange, s *Subscription, ap assetPairs, assets asset.It
 	}
 
 	buf := &bytes.Buffer{}
-	if err := t.Execute(buf, subCtx); err != nil { //nolint:govet // Shadow, or gocritic will complain sloppyReassign
+	if err := t.Execute(buf, subCtx); err != nil {
 		return nil, err
 	}