Remove direct usage of tornado

[blink1073]

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See https://github.com/jupyter-server/jupyter_server/issues/1362 for discussion. This will require a major version change since the loops are public properties.

• [x] Make the app use asyncio.main
• [x] Move the get_event_loop function to jupyter_core (renamed to ensure_event_loop)
• [x] Get unit tests passing
• [x] Get downstreams passing
• [x] Document changes in migration guide

[blink1073]

Needs some direct tests for the new ZMQStream class, including the ability to pass through, such as if stream is not None and not stream.closed().