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.

engine/rpcserver.go

@@ -3103,10 +3103,10 @@ func (s *RPCServer) WebsocketGetSubscriptions(_ context.Context, r *gctrpc.Webso
 		}
 		payload.Subscriptions = append(payload.Subscriptions,
 			&gctrpc.WebsocketSubscription{
-				Channel:  subs[i].Channel,
-				Currency: subs[i].Currency.String(),
-				Asset:    subs[i].Asset.String(),
-				Params:   string(params),
+				Channel: subs[i].Channel,
+				Pair:    subs[i].Pair.String(),
+				Asset:   subs[i].Asset.String(),
+				Params:  string(params),
 			})
 	}
 	return payload, nil