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 `Doctrine\Bundle\DoctrineBundle\Mapping\MetadataFactory` 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 update all these classes to the latest version of the jms metadata project (https://github.com/schmittjoh/metadata) in the future.