Allow sqlite db to be saved in a different location

[ellisonbg]

Problem

We are testing this on an NFS filesystem and finding that the sqlite ystore that is default is struggling to keep up on this filesystem. To resolve this, we are experimenting with putting the sqlite db on a different (and faster) filesystem. We have made changes in our local source to change the location and it seems to resolve the performance issues, but we would like the db location to be configurable.

Proposed Solution

Add a new configurable trait to set the location of the sqlite db.

[ellisonbg]

I should note, when we moved the sqlite db, we began to see this exception raised in the logs:

https://github.com/jupyter-server/jupyter_server_ydoc/blob/v0.1.14/jupyter_server_ydoc/ydoc.py#L124

[davidbrochart]

This is strange, the exception you're mentioning is related to the file ID SQLite database, not the YStore SQLite database.

[davidbrochart]

Add a new configurable trait to set the location of the sqlite db.

There is already a collaborative_ystore_class trait in JupyterLab, that allows to even use another backend for storing Y updates (can be a file, a temporary file, or an SQLite database for now).
To change the location of the SQLite database, you need to subclass SQLiteYStore and override db_path:

from ypy_websocket.ystore import SQLiteYStore

class MySQLiteYStore(SQLiteYStore):
    db_path = "path/to/my/ystore.db"

Then pass this class e.g. at the command line:

jupyter lab --collaborative --LabApp.collaborative_ystore_class=my_module.MySQLiteYStore

[davidbrochart]

The way to pass the YStore class at the CLI is now:

jupyter lab --collaborative --YDocExtension.ystore_class=my_module.MySQLiteYStore