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/subscription/subscription_test.go

@@ -0,0 +1,60 @@
+package subscription
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/thrasher-corp/gocryptotrader/currency"
+	"github.com/thrasher-corp/gocryptotrader/exchanges/asset"
+	"github.com/thrasher-corp/gocryptotrader/exchanges/kline"
+)
+
+// TestEnsureKeyed logic test
+func TestEnsureKeyed(t *testing.T) {
+	t.Parallel()
+	c := Subscription{
+		Channel: "candles",
+		Asset:   asset.Spot,
+		Pair:    currency.NewPair(currency.BTC, currency.USDT),
+	}
+	k1, ok := c.EnsureKeyed().(DefaultKey)
+	if assert.True(t, ok, "EnsureKeyed should return a DefaultKey") {
+		assert.Exactly(t, k1, c.Key, "EnsureKeyed should set the same key")
+		assert.Equal(t, k1.Channel, c.Channel, "DefaultKey channel should be correct")
+		assert.Equal(t, k1.Asset, c.Asset, "DefaultKey asset should be correct")
+		assert.Equal(t, k1.Pair, c.Pair, "DefaultKey currency should be correct")
+	}
+	type platypus string
+	c = Subscription{
+		Key:     platypus("Gerald"),
+		Channel: "orderbook",
+		Asset:   asset.Margin,
+		Pair:    currency.NewPair(currency.ETH, currency.USDC),
+	}
+	k2, ok := c.EnsureKeyed().(platypus)
+	if assert.True(t, ok, "EnsureKeyed should return a platypus") {
+		assert.Exactly(t, k2, c.Key, "EnsureKeyed should set the same key")
+		assert.EqualValues(t, "Gerald", k2, "key should have the correct value")
+	}
+}
+
+// TestMarshalling logic test
+func TestMarshaling(t *testing.T) {
+	t.Parallel()
+	j, err := json.Marshal(&Subscription{Channel: CandlesChannel})
+	assert.NoError(t, err, "Marshalling should not error")
+	assert.Equal(t, `{"enabled":false,"channel":"candles"}`, string(j), "Marshalling should be clean and concise")
+
+	j, err = json.Marshal(&Subscription{Enabled: true, Channel: OrderbookChannel, Interval: kline.FiveMin, Levels: 4})
+	assert.NoError(t, err, "Marshalling should not error")
+	assert.Equal(t, `{"enabled":true,"channel":"orderbook","interval":"5m","levels":4}`, string(j), "Marshalling should be clean and concise")
+
+	j, err = json.Marshal(&Subscription{Enabled: true, Channel: OrderbookChannel, Interval: kline.FiveMin, Levels: 4, Pair: currency.NewPair(currency.BTC, currency.USDT)})
+	assert.NoError(t, err, "Marshalling should not error")
+	assert.Equal(t, `{"enabled":true,"channel":"orderbook","interval":"5m","levels":4,"pair":"BTCUSDT"}`, string(j), "Marshalling should be clean and concise")
+
+	j, err = json.Marshal(&Subscription{Enabled: true, Channel: MyTradesChannel, Authenticated: true})
+	assert.NoError(t, err, "Marshalling should not error")
+	assert.Equal(t, `{"enabled":true,"channel":"myTrades","authenticated":true}`, string(j), "Marshalling should be clean and concise")
+}