Asynchronously gather futures in the background (add a results_in_order util function)

This is a utility that's a mix between `as_completed` and `client.gather()`

`client.gather` returns results in strict order, but if the next result is available we still have to wait for all results
`as_completed` yields results in the order they are completed so you don't have to wait for any future before you can work on them

it would be ideal to have an `results_in_order` function that is like as_completed but with strict ordering