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:

~~~~
#define UV_LOOP_THREADPOOL_FIELDS \
void* wq[2];  \
uv_mutex_t wq_mutex;       \
uv_async_t wq_async;
~~~~