Make http requests async, switch from attohttpc to reqwest

[jcgruenhage]

This is not tested properly yet. I've run cargo clippy and cargo test on
it, but I've not actually tried it out in a real deployment yet. It should
probably be fine though, considering this compiles :P

Does this need a changelog update as well? It already mentions "Manual
(and badly designed) threads have been replaced by async." under
unreleased, so I'd see this as just a small follow-up to that.

[breard-r]

Thank you! I don't think a changelog update is needed for this one, as you noted it's already included in the async part.