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.

cmd/documentation/config_templates/config_readme.tmpl

@@ -6,21 +6,21 @@
 
  + Contains configurations for:
 
-	- Exchanges for utilisation of a broad or minimal amount of enabled
-	exchanges [Example](#enable-exchange-via-config-example) for
-	enabling an exchange.
+	- Enable/Disable Exchanges. [See Example](#enable-exchange-via-config-example)
 
 	- Bank accounts for withdrawal and depositing FIAT between exchange and
-	your personal accounts [Example](#enable-bank-accounts-via-config-example).
+	your personal accounts. [See Example](#enable-bank-accounts-via-config-example)
 
-	- Portfolio to monitor online and offline accounts [Example](#enable-portfolio-via-config-example).
+	- Portfolio to monitor online and offline accounts. [See Example](#enable-portfolio-via-config-example)
 
 	- Currency configurations to set your foreign exchange provider accounts,
 	your preferred display currency, suitable FIAT currency and suitable
-	cryptocurrency [Example](#enable-currency-via-config-example).
+	cryptocurrency. [See Example](#enable-currency-via-config-example)
 
 	- Communication for utilisation of supported communication mediums e.g.
-	email events direct to your personal account [Example](#enable-communications-via-config-example).
+	email events direct to your personal account. [See Example](#enable-communications-via-config-example)
+
+    - Websocket subscription channels. [See Example](#configure-exchange-websocket-subscriptions)
 
 # Config Examples
 
@@ -194,6 +194,38 @@ comm method and add in your contact list if available.
 },
 ```
 
+## Configure exchange websocket subscriptions
+
++ Websocket subscriptions provide a stream of data from an exchange.
+Whilst subscriptions are specific to each exchange, some common examples are: candles, orderbook, ticker and allTrades.
+You can configure any supported channels in your exchange, but most likely you just want to disable some of the defaults, or change the default intervals.
+You cannot configure an empty list of subscriptions, instead set all of the subscriptions to enabled: false.
+
+See the section `exchange.features.enabled.subscriptions` for configuring subscriptions:
+
+```js
+"subscriptions": [
+ {
+  "enabled": true,
+  "channel": "ticker"
+ },
+ {
+  "enabled": false,
+  "channel": "allTrades"
+ },
+ {
+  "enabled": true,
+  "channel": "candles",
+  "interval": "1m"
+ },
+ {
+  "enabled": true,
+  "channel": "orderbook",
+  "interval": "100ms"
+ }
+]
+```
+
 
 ## Configure Network Time Server