[v4] hide access to internal mutex (#185)

* changed embedded mutex to private field

* update ci

* fix tests

subscriber.go

@@ -48,7 +48,6 @@ type Subscriber struct {
 	done         chan struct{}
 	kopts        broker.Options
 	opts         broker.SubscribeOptions
-
 	connected    *atomic.Uint32
 	mu           sync.RWMutex
 	closed       bool