Switch to async for notebook client

[fcollonval]

The foundation of jupyter ydoc is built on top of pycrdt a thin wrapper of Yrs. Unfortunately that library is not thread safe. Therefore when implementing the logic for an agent reacting to document changes, we were facing lost of critical runtime error because the changes and the callback are not necessarily implemented in the same thread.

basically the document was exchanging with the peers through a websocket in a thread when the document itself and the callback on changes were handle in the main thread.

The best workaround while keeping Yrs is to move to an async approach to fake multi-threading. Hence this PR.