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/sharedtestvalues/sharedtestvalues.go

@@ -18,6 +18,7 @@ import (
 	exchange "github.com/thrasher-corp/gocryptotrader/exchanges"
 	"github.com/thrasher-corp/gocryptotrader/exchanges/asset"
 	"github.com/thrasher-corp/gocryptotrader/exchanges/stream"
+	"github.com/thrasher-corp/gocryptotrader/exchanges/subscription"
 )
 
 // This package is only to be referenced in test files
@@ -61,8 +62,8 @@ func NewTestWebsocket() *stream.Websocket {
 		ToRoutine:         make(chan interface{}, 1000),
 		TrafficAlert:      make(chan struct{}),
 		ReadMessageErrors: make(chan error),
-		Subscribe:         make(chan []stream.ChannelSubscription, 10),
-		Unsubscribe:       make(chan []stream.ChannelSubscription, 10),
+		Subscribe:         make(chan []subscription.Subscription, 10),
+		Unsubscribe:       make(chan []subscription.Subscription, 10),
 		Match:             stream.NewMatch(),
 	}
 }