Rely on progress sender for awaiting termination

[rklaehn]

For the store trait, see #56, we have several fns that take a progress emitter and return a future to indicate completion. This might be convenient sometimes, but we have two mechanisms to notify completion - a progress complete or error event and the final result.

We could always indicate completion using the progress stream. There would be events Complete and Error(io::Error). That way the fn can return unit, and the store trait gets more simple. Usage gets slightly more annoying, but we can make a wrapper for that.

[rklaehn]

I am using a naked/unwrapped tokio::sync::mpsc::Sender now, and just use try_send for progress events and send().await for required events.

[rklaehn]

Implemented in the new store