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

* changed embedded mutex to private field

* update ci

* fix tests

subscriber.go

@@ -39,6 +39,7 @@ type consumer struct {
 }
 
 type Subscriber struct {
+<<<<<<< HEAD
 	consumers    map[tp]*consumer
 	c            *kgo.Client
 	htracer      *hookTracer