These changes will make easier for implementors to reuse the event scheduling code

The overall idea is that almost most implementors will have to provide a ScheduledExecutorService and override every and cron methods, so a hierarchy of three classes has been defined accodingly

EventWorfklowScheduler is the top hierarchy and deal with the event part (which relies on Event comsuner), I do not expect this logic to be ever overriden, but who knows ;)
Then ExecutotServiceWorkflowScheduler depends on a provided ScheduledExecutorService, normally implementors that needs a different cron logic than the default one will extend this one and potentially override the every method to provide scheduling persistence (I do not expect after to be override because that one is a single timer that do not require complex handling, but it is also an option)
Finally, DefaultWorkflowScheduler provides the cron logic.