Bypass event_add when using event_base_once() for a 0-sec timeout

Bypass event_add when using event_base_once() for a 0-sec timeout

Some people use event_base_once(EV_TIMEOUT) to make a callback get
called "immediately".  But this is pretty roundabout: it uses the
timeout heap to immediately put the event onto the active queue, when
it could just use event_active.  Additionally, it can lead to
surprising re-ordering behavior.

This patch changes event_base_once so it bypasses event_add() and
called event_active() directly on a pure-timeout event with an empty
timeout.