(This is only available from Windows Vista onwards.)

Instead proactor.Future is now an exception. This substantially reduces
the overhead in the case where blocking is not necessary.

So now we would write things like

    try:
        return context._eventloop.proactor.connect(sock, address)
    except Future as f:
        yield from scheduling.block_future(f)
        return f.result()

The confusion was caused by the fact that select.epoll() uses seconds
for specifying the timeout, whereas select.poll() uses milliseconds.

Thanks to tailhook.

Note that the IOCP proactor does not support ssl sockets or (currently)
ipv6.  Also, there is no kqueue proactor.

To use IOCP first compile by doing

    python setup.py build