Marker interface for those events which should be broadcast to other nodes in a cluster.
Broadcast events should be used sparingly, as they will have a major impact on performance of a cluster.
Where possible, use Bandana or a clustered cache to store data which should be shared between different
nodes, rather than using events to synchronise application state.
The clustered event handler will make sure that events produced within a transaction are not actually
sent to the other cluster nodes until the transaction is successfully committed.