Add async callbacks for long-running commands

[philippjfr]

When starting, stopping or restarting a session or deployment you currently have the option to wait. However starting a deployment can take a couple of minutes and in certain applications (e.g. a dashboard) you don't want to block that long. It would therefore be nice if you could schedule an async callback which is called when the task completes.

In my particular use case this would allow me to show notifications in the dashboard when a task completes.

[mcg1969]

Sorry I haven't responded to this yet. I do think it's a good idea. Let's brainstorm on a good way to implement this. Note that I tried to implement a single "_wait" method and found that I could not reliably do so; there are differences between the different events that one should wait for. But if we do this, we can certainly implement a single approach to specify callbacks, even if the internal wait loop for each is different.