Remove Tornado dependency

[JustAnotherArchivist]

Tornado used to run the show here, but it's only used for very few things now thanks to asyncio. Let's evaluate whether any of them really need Tornado, although I'd expect it can all be done without much fuss with the standard library. Getting rid of the dependency would help maintainability; we're currently stuck two major versions behind upstream.

[HeliosLHC]

I upgraded my wpull fork to the latest Tornado 6 a couple months back. I do have to agree it would be better if we perhaps remove it.