MINOR: event_hdl: provide event->when for advanced handlers

MINOR: event_hdl: provide event->when for advanced handlers

For advanced async handlers only
(Registered using EVENT_HDL_ASYNC_TASK() macro):

event->when is provided as a struct timeval and fetched from 'date'
haproxy global variable.

Thanks to 'when', related event consumers will be able to timestamp
events, even if they don't work in real-time or near real-time.
Indeed, unlike sync or normal async handlers, advanced async handlers
could purposely delay the consumption of pending events, which means
that the date wouldn't be accurate if computed directly from within
the handler.