Asyncio progress reporting

I'd like to send progress reports through a websocket.
Currently I can't find a nice way to achieve that.

Could we implement an interface similar to the following?

```python
from distributed.diagnostics import Progress

future = client.compute(...)

async for processed_key, completeness in Progress(future):
logging.info(processed_key)
websocket.send(completeness)
```

Somewhat related to https://github.com/dask/distributed/issues/1020