fix: prefetcher leaks one message at max_tasks_to_execute boundary

[PythonFZ]

Fix using max_tasks_to_execute causes the prefetcher to pop tasks from the queue it has no intention of processing.

Closes#528.

[s3rius]

Hello. Thanks for your contribution. However, I'd love to move try inside the while loop, because in case of any error prefetcher will stop working completely which will make worker to hang. It might happen, because of asyncio.wait for example.

[PythonFZ]

Hello. Thanks for your contribution. However, I'd love to move try inside the while loop, because in case of any error prefetcher will stop working completely which will make worker to hang. It might happen, because of asyncio.wait for example.

That is a good point, I added a broad try ... except Exception in 592b167 and a test for transient listening failures in adeb379

Does that work for you or are there any concerns with calling iterator = self.broker.listen() in the exception handling?