WIP Allow usage of Symfony EventDispatcher instead of Doctrine EventManager

[GromNaN]

Q	A
Type	feature
BC Break	no
Fixed issues	-

Summary

Symfony EventDispatcher is a very close to Doctrine EventManager in features (add/remove listener, and dispatch an event using a name and an event object).

Why not using sticking to PSR-14: Event Dispatcher?

The PSR EventDispatcher interface method dispatch(object $event) relies on the object class. But in Doctrine ODM, the same event class LifecycleEventArgs is used for multiple events (postUpdate, postRemove, postUpdate, prePersist, preRemove, preUpdate).
We can decide to create a distinct class for each event and use the PSR EventDispatcherInterface. With a map of class names to event names.

The event names are very generic, it would be possible to prefix them with doctrine.mongodb. to prevent any conflict if the same event dispatcher is used for other events outside of Doctrine.

Livecycle callbacks

The livecycle callbacks are not using the event manager, they are called independently in LifecycleEventManager.