A base class for events related to the cluster.
This class exists primarily to simplify writing event types. Plugins should not listen for this low-level base
class; they should listen for specific subclasses.
Cluster node events are not part of the
ApplicationEvent hierarchy. Most cluster
node events happen in response to system-level actions, like new nodes joining or existing nodes departing, rather than
happening in response to user actions, so they have their own hierarchy.
- the timestamp when the event was raised