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!
Wouldn't it be less fuzz to still store `err` here and update `req->retcode` at `complete`?