shared debug sessions

[akosyakov]

Debug sessions should survive between user sessions. A user should be able to access all debug sessions running by the backend.

[akosyakov]

That's important to implement to avoid loosing sessions on each reconnect. It will be quite straightforward for native extensions. But for VS Code extensions the debug process is forked from the plugin host process, so it would be gone on the reconnect. We need to make sure instead that even debug session from VS Code extensions are forked from the Theia backend process. cc @tolusha for shared remote sessions in Che then you will need to collect them somehow from the main Theia instance.