Migrate YRoom classes to traitlets, fix server extension stopping

[dlqqq]

Description

• Makes YRoomManager a logging configurable, set by the ServerDocsApp.yroom_manager_class trait.

• Makes YRoom a logging configurable, set by the YRoomManager.yroom_class trait.

• YRoomFileAPI, YRoomEventsAPI, and YjsClientGroup have all been made logging configurable, set by the YRoom.file_api_class, YRoom.events_api_class, and YRoom.client_group_class traits respectively.

• Adds the YRoomFileAPI.poll_interval float trait, which configures how frequently the YRoomFileAPI writes unsaved changes to disk, checking for in-band & out-of-band changes before doing so.

• Fixes a regression introduced by Allow YRoom to be restarted and allow multi-room kernels #117, where the server extension would not shut down properly.

Technical details

#117 refactored the YRoom task to make its stop() method fully synchronous. Each room's content would be saved in a background task. Since YRoom.stop() & YRoomManager.stop() were made synchronous, the ServerDocsApp.stop_extension() method was also made synchronous.

However, that change led to a race condition where the ContentsManager shut down before each room's content was saved to disk. The regression has been fixed by adding a YRoom.until_saved awaitable property that is only awaited when the server shuts down. The ServerDocsApp.stop_extension() method is now asynchronous again.

Reviewer guidance

• Verify that the configurable traits work
• Verify that the server extension stops gracefully
• Verify file contents are saved after the server extension stops

[dlqqq]

@Zsailer Thank you for the feedback. I've gone through the logging configurables added by this PR and drastically simplified all of their constructors. Each only takes 1-2 arguments now. 👀

However, I wasn't able to remove every __init__() or make every instance attribute a trait because there are scenarios where traitlets breaks type safety. Here is what happened when I turned YRoom.file_api into a traitlets.Instance trait:

Even though I had specified klass=YRoomFileAPI, the type checker is unable to resolve the type of instance traits. I believe this is the same issue causing self.log to be unresolved in LoggingConfigurable instances.

I don't think that using traitlets should come at the cost of type safety, especially if instance attributes work for now & there is not a clear need to use private traits. I've opened a follow-up issue for more discussion & to allow these changes to be addressed in a future PR: #123.

[dlqqq]

Same issue is occurring on ServerDocsApp.yroom_manager after making it an Instance trait:

[Zsailer]

Thanks @dlqqq! Great work here. Let's merge and use #123 for further discussion/iteration. I think any further pushing on traitlets is low priority :) We're getting configuration here that we want, and that's a big win!