fix: use spawn_callback to avoid blocking socket ping/pongs

[zealenth]

References

When using progressive rendering to render reports that take more than 90s, the report hangs indefinitely.

Reproduced with a cell

import time
time.sleep(91)
print('hi')

Code changes

I tracked this down to tornadoweb/tornado#2021. In our case, the execution handler's on_message method blocks any ping/pong requests from being received, resulting in the websocket being closed after 90s

I've taken their recommend approach and offloaded the notebook handling to spawn_callback.

User-facing changes

Slow reports that previously hung will now render.

Backwards-incompatible changes

[github-actions]

👈 Launch a Binder on branch zealenth/voila/fix%2Fprogressive-rendering%2Fwebsocket-timeouts