Graceful server stopping

[akx]

Describe what this pull request is trying to achieve.

This is a part of #10291 (test overhaul).

The point is to be able to gracefully shut down the server (e.g. to be able to save code coverage information, for testing).

Additional notes and description of your changes

• the need_restart mechanism is changed to server_command (with "stop" and "restart" being valid values)
• there's a new state.request_restart() API for setting this, which deduplicates a couple copy-pasted segments
• instead of having the webui() poll for need_restart every 0.5 seconds, there's a new wait_for_server_command() API
• finally, there's a new option to add a _stop route that will cause server_command to be set to "stop". This is so that the CI process can start the server in the background and then ask it to quit when tests are finished.
  • This could be changed to a signal such as SIGUSR1 too, if the option and HTTP route is too much.
  • I figured an option would be better than just always enabling a remote way to make the server die.

Environment this was tested in

• OS: Windows, macOS
• Browser: Chrome
• Graphics card: Irrelevant