Add a default lock mode to the EntityManager

Following [this discussion](https://groups.google.com/forum/#!topic/doctrine-dev/xKGzQcDkilE) on the mailing list, this is a first draft of a proposal to introduce a default lock mode for all entities loaded through an EntityManager.

At the moment, there is no way to set a lock mode for the following use cases:
- Proxies obtained through `getReference()` and then initialized
- Entities lazy-loaded through traversal of associations

This proposal introduces the idea of a *default* lock mode, which can be set at runtime when all reads in a transaction should be locking.

It works this way:

    $em->beginTransaction();
    $em->getConfiguration()->setDefaultLockMode(LockMode::PESSIMISTIC_WRITE);
    
    // load entities from EntityManager, Repositories or DQL, traverse associations, etc.
    // all these entities will be loaded with the given lock mode
    
    $em->commit();
    $em->getConfiguration()->setDefaultLockMode(null);

I have successfully tested it with the following use cases:
- `EntityManager::find()`
- `EntityRepository::findBy()`
- DQL queries
- Proxies
- Lazy-loaded collections through OneToMany and ManyToMany associations

Before moving forward and writing proper unit tests, I'm looking for your feedback on this proposal. Is this a concept you would be happy to integrate in Doctrine?

If yes, I have a doubt as regards to where the default lock mode should be set: @Ocramius suggested to set it on the `Configuration`; this looked reasonable at first glance, and I have implemented it this way for now. It feels a tiny bit wrong though now that I see it, as I feel like the contents of the Configuration should should only be set during bootstrapping, rather than being set and reset in the controllers as in the example above. I might be wrong obviously.

My suggestion would be to move the default lock mode to the EntityManager, so that the code would become:

    $em->beginTransaction();
    $em->setDefaultLockMode(LockMode::PESSIMISTIC_WRITE);

Happily waiting for your feedback!