Make aria2 wrapper and Downloader thread safe/compliant.

[mgautierfr]

See kiwix/kiwix-desktop#87 (comment)

aria2 wrapper use a lock to prevent concurrent access to the curl handle. This create a bottleneck as we can do only one request at the time. If one request takes time (and starting a download does on window) all other requests are delayed (and blocked).

We should make aria2 wrapper thread compliant and also make Downloader thread safe to allow a real multi thread usage of Downloader

(Not sure we need to make Download threadsafe. We can simple state that methods on one Download are not threadsafe)

[kelson42]

Seems pretty urgent if we want that kiwix/kiwix-desktop#87 works fine.