Messages randomly not delivered

Hi!

I'm using elasticmq with Go and currently this is my pooling implementation:

func listenForLocalSQSEvents(urls []string) {
	ctx := context.Background()
	sess := session.Must(session.NewSessionWithOptions(session.Options{
		Config: aws.Config{
			CredentialsChainVerboseErrors: aws.Bool(true),
			Endpoint:                      aws.String(env.GetString("ELASTICMQ_URI")),
			Region:                        aws.String("ELASTICMQ_REGION"),
		},
	}))
	svc := sqs.New(sess)
	for _, url := range urls {
			go func(url string) {
				for {
					log.Printf("requesting at: %v", time.Now())
					msgResult, err := svc.ReceiveMessage(&sqs.ReceiveMessageInput{
						AttributeNames: []*string{
							aws.String(sqs.MessageSystemAttributeNameSentTimestamp),
						},
						MessageAttributeNames: []*string{
							aws.String(sqs.QueueAttributeNameAll),
						},
						QueueUrl:            aws.String(url),
						MaxNumberOfMessages: aws.Int64(10),
						WaitTimeSeconds:     aws.Int64(5),
					})
					if err != nil {
						panic(err)
					}
					log.Printf("msgResult: %+v", msgResult.String())
					if err := eventsLambda.Consume(ctx, msgResult); err != nil {
						log.Printf("local sqs consumer error: %v", err)
						continue
					}
				}
			}(v)
	}
}

The problem is, some messages are never delivered and some are delivered with no delay at all.
Using the UI I noticed that the messages which are never delivered increase the counter on the Approximate number of not visible Messages column.

No repository file or test is named. Start by reproducing the issue with the supplied Go AWS SQS polling loop and inspect ElasticMQ's receive and visibility handling alongside the reported UI counter. Done means the cause of messages becoming not visible without delivery is identified and the behavior is covered by a focused regression check.