Extend event handles for mult-registry and version support

[christian-pinto]

The redfish event handler groups single events in classes. We should re-engineer the current events plugin to reflect this grouping, to help further development towards supporting the entire registry. Also, the plugin should have the ability of invoking specific message handlers from different versions of the standard for backward compatibility(e.g., supporting legacy hardware/agents).

[christian-pinto]

Upon receiving an event we should validate:

1. validate the event schema is correct
2. existing event_type
3. Valid eventIdentifier (next in sequence, not seen, in range)

For the eventIdentifier a possible implementation is to have a rolling value for each Agent that we reset every X events.