2.x branch and target for core inclusion. Move extra stuff to sub-modules.

If I were to re-write HTTPRL to be object code, so it is pluggable (#64866: Pluggable architecture for drupal_http_request()), and have the threading callbacks/background callbacks be implemented via the hook system (using sub-modules) I think it would be suitable for core. Basic code would be as small as possible and should be extensible as I've got a threading library built on top of it.