Make *EventArgs generic

[derrabus]

Implementations of the persistence contracts usually use a more specific contract for their own ObjectManager implementation, like EntityManagerInerface for the Doctrine ORM.

This PR allows implementations to be more specific about the ObjectManager object returned by event objects. That in turn should help us to provide more precise types in the ORM.

[malarzm]

FWIW there's a base event in ODM to force DocumentManager: https://github.com/doctrine/mongodb-odm/blob/4cc90c39f8169078b0c6825384ccfea82559535d/lib/Doctrine/ODM/MongoDB/Event/ManagerEventArgs.php#L17 or sometimes we were just adding that method to our event classes. I'm not sure if we'd be removing those methods with your PR merged, but surely can slap few more pslam annotations to classes :)

[derrabus]

We have kind-of the same in ORM as well:

https://github.com/doctrine/orm/blob/1ffb9152f76bd3d332d5275bd2272d3984bcf3d6/lib/Doctrine/ORM/Event/LifecycleEventArgs.php#L28-L36

In your case, this PR would allow you to remove the assert($dm instanceof DocumentManager); hack.

[malarzm]

Yeah, removing an assert is always welcome :) Will you be considering dropping getEntityManager() methods from ORM's events or are they to stay as well?

[derrabus]

I think, we should keep them because to an ORM user, the term "entity manager" is probably more familiar than "object manager".