Workaround for libev handle caching bug on windows

The windows socket api uses handles, not fds. Libev caches the handle
associated with an fd, and uses this handle when updating the (also cached)
handle set that goes into select(). When an fd is closed and subsequently
re-used before the event loop returns to libev, libev fails to detect
properly that the handle changed. And even if it does, the cached handle of
the closed socket is overwritten by the new handle, so by the time libev
tries to update the select fdset it has forgotten which handle to remove
from it. This is solved by a simple hook ev_fd_closed that makes it clear
its caches before the fd is re-used.