Some busy indicators not firing

[jcheng5]

With the examples/busy_indicators/app.py app, on Windows, I'm pretty reliably able to get only one out of the four plots to show progress.

https://jam.dev/c/fce934dd-d6de-4514-9ae5-184bbc74684d

I can see from the Python console that Shiny is sending the right messages, but some of them are getting batched up, similar to #1373, except in this case uvloop is not installed. Also, if I use --reload it doesn't happen.

[jcheng5]

I wonder if we need to get the I/O event loop and the session event loop onto separate threads, like we did in R.

[jcheng5]

I followed the call to conn.write() with a debugger down to here:
https://github.com/python/cpython/blob/5b88d95cc542cf02303c6fe0e8719a93544decdb/Lib/asyncio/proactor_events.py#L363-L374

When the first branch is taken, the message goes right away. But in the repro case, that's only true for the first progress message; the second message goes to the second branch, and all the remaining ones go to the third branch, and none of those get sent until computation is complete.

[jcheng5]

I filed a PR that will fix this. Still doesn't feel awesome that our I/O is tangled up with the app logic like this, I have a feeling this will come back with insert_ui or custom messages.