lib/hub: Event.wait should return its internal flag

'pydoc gevent.event.Event' says

> Help on method wait in gevent.event.Event:
>
> gevent.event.Event.wait = wait(self, timeout=None) unbound gevent.event.Event method
>     Block until the internal flag is true.
>     If the internal flag is true on entry, return immediately. Otherwise,
>     block until another thread calls :meth:`set` to set the flag to true,
>     or until the optional timeout occurs.
>
>     When the *timeout* argument is present and not ``None``, it should be a
>     floating point number specifying a timeout for the operation in seconds
>     (or fractions thereof).
>
>     Return the value of the internal flag (``True`` or ``False``).

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>