UPGRADE_TO_2_2

# EntityManager#getPartialReference() creates read-only entity

Entities returned from EntityManager#getPartialReference() are now marked as read-only if they
haven't been in the identity map before. This means objects of this kind never lead to changes
in the UnitOfWork.

# Fields omitted in a partial DQL query or a native query are never updated

Fields of an entity that are not returned from a partial DQL Query or native SQL query
will never be updated through an UPDATE statement.

# Removed support for onUpdate in @JoinColumn

The onUpdate foreign key handling makes absolutly no sense in an ORM. Additionally Oracle doesn't even support it. Support for it is removed.

# Changes in Annotation Handling

There have been some changes to the annotation handling in Common 2.2 again, that affect how people with old configurations
from 2.0 have to configure the annotation driver if they don't use `Configuration::newDefaultAnnotationDriver()`:

    // Register the ORM Annotations in the AnnotationRegistry
    AnnotationRegistry::registerFile('path/to/Doctrine/ORM/Mapping/Driver/DoctrineAnnotations.php');

    $reader = new \Doctrine\Common\Annotations\SimpleAnnotationReader();
    $reader->addNamespace('Doctrine\ORM\Mapping');
    $reader = new \Doctrine\Common\Annotations\CachedReader($reader, new ArrayCache());

    $driver = new AnnotationDriver($reader, (array)$paths);

    $config->setMetadataDriverImpl($driver);