Much faster method for getting Metadata of a class

Since commit [cc6673](https://github.com/symfony/symfony/commit/cc66739f3d0fb50fba673f921d971bf62dc1b2f9) resolving issue [#2688](https://github.com/symfony/symfony/issues/2688), generating entities with the `app/console doctrine:generate:entities` command have become very slow, especially with many entities.

The problem comes from the `getMetadataForClass` method of the `Doctrine\Bundle\DoctrineBundle\Mapping\MetadataFactory` class which was looking for the right class after getting the metadata of all classes.
I changed the implementation so that only the metadata of the given class is retrieved.
Generating my entities is now about 12 times faster in my project.

~~Maybe we should consider updating all these classes to the latest version of the jms metadata project (https://github.com/schmittjoh/metadata) in the future.~~

edit: removed last sentence