Binance: Subscribe/unsubscribe response handling (#1444)

* Binance: Fix subscription failures ignored

* Testing: Fix race on shared config singleton

* Config: Privatise Global config var

We should *either* use a private var *or* use an accessor, but it
doesn't make sense to mix paradigms.
Since GetConfig() is well established this instead removes the limited uses of direct public access and adds a Setter

* Zip: Fix test failure on http mocks

engine/engine.go

@@ -64,7 +64,7 @@ func New() (*Engine, error) {
 	newEngineMutex.Lock()
 	defer newEngineMutex.Unlock()
 	var b Engine
-	b.Config = &config.Cfg
+	b.Config = config.GetConfig()
 
 	err := b.Config.LoadConfig("", false)
 	if err != nil {