exchanges/websocket: Implement subscription configuration (#1394)

* Websockets: Move Subscription to its own package

This allows the small type to be imported from both `config` and from
`stream` without an import cycle, so we don't have to repeat ourselves

* Subs: Renamed Currency to Pair

This was being mis-used through much of the code, and since we're
already touching everything, we might as well fix it

* Websockets: Add Subscription configuration

* Binance: Add subscription configuration

* Kucoin: Subscription configuration

* Simplify GenerateDefaultSubs
* Improve TestGenSubs coverage
* Test Candle Sub generation
* Support Candle intervals
* Full responsibility for formatting Channel name on GenerateDefaultSubs
  OR consumer of Subscribe
* Simplify generatePayloads as a result
* Fix test coverage of asset types in processMarketSnapshot

* Exchanges: Abstract ParallelChanOp

* Tests: Generic ws mock instances

* Kucoin: Fix intermittent conflict in test currs

Use isolated test instance for `TestGetOpenInterest`.

`TestGetOpenInterest` would occassionally change pairs before
GenerateDefault Subs.

exchanges/exchange_types.go

@@ -14,6 +14,7 @@ import (
 	"github.com/thrasher-corp/gocryptotrader/exchanges/protocol"
 	"github.com/thrasher-corp/gocryptotrader/exchanges/request"
 	"github.com/thrasher-corp/gocryptotrader/exchanges/stream"
+	"github.com/thrasher-corp/gocryptotrader/exchanges/subscription"
 )
 
 // Endpoint authentication types
@@ -149,8 +150,9 @@ type WithdrawalHistory struct {
 // Features stores the supported and enabled features
 // for the exchange
 type Features struct {
-	Supports FeaturesSupported
-	Enabled  FeaturesEnabled
+	Supports      FeaturesSupported
+	Enabled       FeaturesEnabled
+	Subscriptions []*subscription.Subscription
 }
 
 // FeaturesEnabled stores the exchange enabled features