Warning about changed file should not happen in RTC

[davidbrochart]

Description

In RTC, the frontend should not warn about a file changed in the backend, this should only happen when RTC is not enabled.
Indeed, in RTC when a file is changed in the backend, it should result in the shared document being cleared and reset to the content of the file (the backend acts as just another peer). But I can sometimes see this:

Reproduce

I'm currently testing RTC with a simulated slow network, and I can see this behavior very easily. I also saw it on Binder. But on a local network it is very difficult to trigger.

Expected behavior

This warning should not be shown.

Context

• Operating System and version: Linux Ubuntu 22.04
• Browser and version: Firefox 119.0.1
• JupyterLab version: 4.1.0b0
• jupyter-collaboration: 2.0.0

[davidbrochart]

This was part of #170.