refactor downloading

The way it was done was so horrid, it was not only impossible to debug,
with TODO comments saying it sucks, and other shit like that, but it was
also just full of data races. You can rest easy however - I left most of
the data races in there <3 For nostalgia (totally not because it's a
massive pain to fix that).

We now do single-threaded download, which can not only saturate my 100
Mbit/s line without any hickups, it can also go up to ~600000 Mbit/s for
localhost transfers :) So I think it's fine.

src/GameStart.cpp

@@ -64,7 +64,6 @@ std::string GetGamePath() {
     std::string Ver = CheckVer(GetGameDir());
     Ver = Ver.substr(0, Ver.find('.', Ver.find('.') + 1));
     Path += Ver + "/";
-    info("Game user path: '" + Path + "'");
     return Path;
 }
 #endif