Bug: Worker with ZeroMQ does not work on Windows with default configuration

[raidzin]

Asynchronous ZeroMQ does not work on Windows with default asyncio.WindowsProactorEventLoopPolicy.

It raises:

RuntimeError: Proactor event loop does not implement add_reader family of methods required for zmq. zmq will work with proactor if tornado >= 6.1 can be found.
Use `asyncio.set_event_loop_policy(WindowsSelectorEventLoopPolicy())` or install 'tornado>=6.1' to avoid this error.

If I install tornado as required by the exception, then the worker starts, but there is a warning that I need to configure the eventloop.

RuntimeWarning: Proactor event loop does not implement add_reader family of methods required for zmq. Registering an additional selector thread for add_reader support via tornado. Use `asyncio.set_event_loop_policy(WindowsSelectorEventLoopPolicy())` to avoid this warning.        
  self._get_loop()

Adding asyncio.set_event_loop_policy(WindowsSelectorEventLoopPolicy()) to the application code has no effect for worker

[raidzin]

@s3rius, I see two ways to solve this problem:

1. We can add tornado as a platform-specific dependency when we use the optional zmq dependency. With this, the worker will start, but the warning will remain
2. We can add a check that pyzmq is installed and os.name == 'nt' and then change the default EventLoopPolicy to WindowsSelectorEventLoopPolicy when worker starts

To be honest, I can't say exactly what side effects the second option might have, but I tested on pure pyzmq and it worked without a tornado