Modernize event loop behavior

[blink1073]

Based on discussion in https://github.com/python/cpython/issues/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.

This ensures we install an event loop before launching our apps, so we should not see any more issues like https://github.com/jupyter/notebook/issues/6721.

Additionally, it adds a new class, JupyterAsyncApp that runs initialize_async() and start_async() in an event loop.

Based on results in https://github.com/ipython/ipykernel/pull/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 ensure_event_loop so we can override _prefer_selector_loop in jupyter_server apps to prefer the selector loop.

Based on failures seen in #383, we add tests to ensure we are not interfering with existing event loop behavior.

[blink1073]

Could you name it something other than ‘get_event_loop’, to avoid confusion with the asyncio function by that name? Maybe ‘ensure_event_loop’?

Done, thank you!