azure-sdk-for-net: [BUG] ServiceBus MessageReceiver: can't process more than prefetchCount messages simultaneously.

Describe the bug We are using ServiceBus to trigger long-running tasks processing. We can’t process more than prefetchCount messages simultaneously, because MessageReceiver.ReceiveAsync stops returning any messages.

Exception or Stack Trace N/A

To Reproduce Send N messages to ServiceBus. Create new MessageReceiver with options: receiveMode: ReceiveMode.PeekLock, prefetchCount: M, where M < N. Call MessageReceiver.ReceiveAsync in a loop and don’t complete any messages. Only M message will be returned in a total.

NOTE: MessageReceiver.ReceiveAsync will return new messages after LockToken expiration or completing message.

Code Snippet

        public async Task TestWithoutComplete()
        {
            int messageCount = 100;

            var queueClient = new QueueClient(ConnectionString, QueueName);
            var messagesToSend = Enumerable
                .Range(1, messageCount)
                .Select(i => new Message()).ToList();
            await queueClient.SendAsync(messagesToSend);

            var receiver = new MessageReceiver(ConnectionString, QueueName, receiveMode: ReceiveMode.PeekLock, prefetchCount: messageCount / 2);
            var receivedMessages = new List<Message>();

            while (messageCount != receivedMessages.Count)
            {
                var newMessages = await receiver.ReceiveAsync(messageCount);
                if (newMessages != null)
                {
                    receivedMessages.AddRange(newMessages);
                }
            }
        }

Expected behavior Receive message until ServiceBus Queue is empty.

Screenshots N/A

Setup (please complete the following information):

• OS: Windows 10 or Service Fabric
• IDE : Microsoft Visual Studio 2019
• NuGet package: Microsoft.Azure.ServiceBus 3.4.0

Additional context It was tested on different numbers of messages and prefetchCount.

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