Use more async and allow parallel downloads

[themadprofessor]

No description provided.

[auyer]

Hi! Thank you for your contribution. First of all, the parallel download is really nice.

However there are a few thing we need to check.

1. In my tests, the validation of the latest Proton version failed a couple times, and was successful right after. Not sure what caused this behavior. Adding a test case for this would be very interesting.
   
2. Aren't we overusing async ? There are a few branches that I see no advantage in being async. For instance, the detect_installations and check_if_exists functions. They are two filesystem related functions that are both simple and fast. Making them non-blocking wont make the app any faster. And when they are called, nothing else is happening in the software.

Please, tell me what you think. Thanks.

[themadprofessor]

That should be the validation fixed.

In general, calling blocking calls within async blocks is not a good idea since it can block multiple futures from progressing. The blocking calls might be fast, but they could be slow. Using the non-blocking versions might not make it any faster, but it is more correct and would prevent a single slow I/O from blocking progress.

[auyer]

It looks good now, and it works vary well!
I could not reproduce the error anymore. I think you nailed it.
Thanks!