Invoked upon lost of event or events.
That event lost can be both a real or a possible event lost situation.
In real event lost situation we can be sure that a sequence of events
are missing and a recovery can be tried but in possible event lost
situation we cannot be sure whether or not there is an event lost, for
example, after a node is killed suddenly, we cannot be sure that all
events on the killed node are sent and received by query caches.