You could add two lines to your code above to achieve your goal.

...
async{
...
}
...
[download]

Of course, a complete solution would add a few more lines in order to terminate slow or absent mirrors. And a couple (2 or 3) more to share the results of the asynchronous calls with the main thread of the code.

The total absence of the word "threads" from your question and responses suggests that you will not consider such a solution...and I've gotton out of the habit of expending time producing and testing solutions that will likely simple be ignored. But for the problem you are trying to solve, threads is the simplest, fastest, easiest to understand solution.

It is also the case that I am not currently in a position to offer a tested solution, and unfortunate that even those here that do not dismiss threads as a viable solution, rarely seem to offer code.

C'est la vie.