Shutting down lots of things in the running tab seems to block JLab

[ellisonbg]

Have seen this a number of times in recent JupyterLab demos in the past week. If you have a bunch of terminals and kernels running, and then use the running tab to quickly shut them all down, it seem to block JupyterLab. More specifically, it seems that file browser navigation changing directories is blocked until all those operations finish. Unfortunately, it doesn't do this all the time, so may be a bit difficult to chase down.

[jasongrout]

This may be covered by #4724, which is ambitious about rewriting the running panel.