Memory Leak when handler fails

[srpeirce]

We have observed a memory leak that seems to be caused when a message fails to process.

In failure scenarios we rely on the peek lock timeout, but we do not call MessageReceiver with CompleteAsync, DeadletterAsync, DeferredAsync etc, so the message is never disposed.

https://github.com/Azure/azure-service-bus-dotnet/blob/master/src/Microsoft.Azure.ServiceBus/Core/MessageReceiver.cs

I initially thought that we could call DeferredAsync but that will put the message back on the queue and instantly be processed.

Instead I think it might be better to clone the message and re-send it after a specified time.