Use supported event loop behavior

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Based on discussion in python/cpython#100160, we should not be relying on asyncio.get_event_loop, since they are in fact going to make it an alias for asyncio.get_running_loop.

As of #381, we are starting the event loop before instantiating our apps, so we should not see any more issues like jupyter/notebook#6721.

Based on results in ipython/ipykernel#1184, we'll want to normally use the default event loop on Windows, unless we know we're running a tornado app. This PR adds a prefer_selector_loop option to get_event_loop so we can override _prefer_selector_loop in jupyter_server apps to prefer the selector loop.

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I'm going to close this in light of #385. After we fix #384 and release, I'll make a new PR that ensures we are handling the problems raised in #383.