An event in `events` collection should be auto generated

If we give a definition to an event is that "A group of `records` which occur simultaneously in the same period of time at the same or nearby location, we could achieve the event auto-generation by just grouping `records` (according to the definition) and _**count**_ them up. If _**count**_ value is greather the given _**threshold**_, then, generate a new event.