Exit read, write threads on web server shutdown

[joshhead]

Proposed fix for #29

I found several potential reasons why child threads were not exiting.

1. thread_stop_event is read but never written
2. queue.Queue.get called in blocking mode without a timeout, preventing write thread from exiting
3. Web socket requests can restart threads. Not sure if this is strictly related, but it seems simpler to account for the threads if they are started once.
4. asyncio.queue.get is called with no timeout in the websocket handler, preventing it from exiting (it seems)

See also these notes on aiohttp graceful shutdown: https://docs.aiohttp.org/en/v0.22.4/web.html#aiohttp-web-graceful-shutdown

Address the above with the following changes

• Start read and write threads in on_startup instead of on websocket connection
• Add shutdown handler to close websockets and set stop event
• Add timeout to queue get() so that reader can check whether it needs to exit

Also

• Add sleep when retrying serial connect to reduce console noise and CPU usage

[teejusb]

Thanks for this and the documentation + opening issue. One very minor stylistic change and I'll merge!

[teejusb]

Thanks!