[watermill-kafka] `Subscriber.Close()` seems not concurrent safe

[xuyang2]

kafka.Subscriber.Close() seems not concurrent safe. close(s.closing) may be executed multiple times when called concurrently, causing panic: close of closed channel .

https://github.com/ThreeDotsLabs/watermill-kafka/blob/9242094c8fd755f501295c65515f2d47bb122962/pkg/kafka/subscriber.go#L468

func (s *Subscriber) Close() error {
	if s.closed {
		return nil
	}

	s.closed = true
	close(s.closing)

see alsao:

• https://github.com/ThreeDotsLabs/watermill-amqp/blob/f8efc8147ada84ac953420f042f87995168a829a/pkg/amqp/subscriber.go#L42-L45
• https://github.com/ThreeDotsLabs/watermill-amqp/blob/f8efc8147ada84ac953420f042f87995168a829a/pkg/amqp/subscriber.go#L78-L81
• https://github.com/ThreeDotsLabs/watermill-nats/blob/1a6d735bb37b3f97ab822abacc233428a56041e6/pkg/nats/subscriber.go#L385-L386