allow publishing while handling a DomainEvent & dispatch events from subclasses to the subscribers of parent classes

[mbenabda]

As Vaugh Vernon said in #6 :

The DomainEventPublisher must protect itself from the subscribers. For
example, when a subscriber's handle() method has been called back on, the
handle() method could attempt to register another subscriber. Even though
it's all happening on a single thread, that would cause a
ConcurrentModificationException on the collection.

Nevertheless, it should be possible to publish an event from inside a subsciber's handle() method, to propagate whatever side effects on the domain model the parent DomainEvent handling might produce.