zmfilter: Send message for events that are still ongoing

Currently, a message (ZM_OPT_MESSAGE) is only sent after an Event has closed. This means that ongoing Events are not reported - which might be a problem for 'long' Events. 

This patch reports ongoing Events to "message" filters only (other filters are not affected). 
It allows configuring a background message filter to achieve "semi-instant" notification - where any new Event will be reported within the FILTER_EXECUTE_INTERVAL - even Events that last (much) longer than that.

As such, this patch also brings the ZM_OPT_MESSAGE behaviour more in line with its documentation, which states "This will allow you to be notified of events as soon as they occur".