unix, windows: use the same threadpool implementation

UNIFY ALL THE THINGS!

With this patch we'll use a single threadpool implementation, the one that we were using on the Unix side. When changes are done to it both platforms will benefit, thus reducing the maintenance burden.

The first visible advantage is that `uv_cancel` now also works on Windows!
You mean all threadpool related fields? Could be, but I thought I'd do that later since I want to unify the structures more (use QUEUEs as well on Windows) and I'd do it in a single shot.