David Brochart

### Problem Currently the default (async) file manager allows to [save](https://github.com/jupyter-server/jupyter_server/blob/3fbf07e57c33b6c536edec730601678fbab45188/jupyter_server/services/contents/filemanager.py#L941) a file and to [read](https://github.com/jupyter-server/jupyter_server/blob/3fbf07e57c33b6c536edec730601678fbab45188/jupyter_server/services/contents/filemanager.py#L880) it concurrently. If this happens, the read file content will probably not consist of...

Added TaskFactory cancel_all_tasks() and wait_all_tasks_finished()

2

This PR is an alternative to #115. Instead of cancelling/waiting for individual tasks, it creates a sub-task group that it can cancel and recreate. The drawback is that it adds...