Fix race condition in notebook kernel association

[msujew]

What it does

There's a race condition in the notebook kernel association code. Whenever you open a notebook, Theia will automatically try to associate a kernel to it. However, if this notification arrives in the plugin host before the notebook open notification arrives, it will not be able to associate the kernel to the notebook - this effectively disables all cell execution for the specific notebook.

How to test

The issue can be fairly reliably reproduced by:

1. Open a notebook and select a kernel for it
2. Just reload the application, with the notebook being open

Previously, notebook cells weren't executing afterwards. This change should alleviate that issue.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[jonah-iden]

Thanks for the fix, i'll take a look tomorrow

[jonah-iden]

Works great and looks good to me 👍