UPGRADE_TO_2_2

# ResultCache implementation rewritten

The result cache is completely rewritten and now works on the database result level, not inside the ORM AbstractQuery
anymore. This means that for result cached queries the hydration will now always be performed again, regardless of
the hydration mode. Affected areas are:

1. Fixes the problem that entities coming from the result cache were not registered in the UnitOfWork
   leading to problems during EntityManager#flush. Calls to EntityManager#merge are not necessary anymore.
2. Affects the array hydrator which now includes the overhead of hydration compared to caching the final result.

The API is backwards compatible however most of the getter methods on the `AbstractQuery` object are now
deprecated in favor of calling AbstractQuery#getQueryCacheProfile(). This method returns a `Doctrine\DBAL\Cache\QueryCacheProfile`
instance with access to result cache driver, lifetime and cache key.


# 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 absolutely 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);


# Scalar mappings can now be ommitted from DQL result

You are now allowed to mark scalar SELECT expressions as HIDDEN an they are not hydrated anymore.
Example:

SELECT u, SUM(a.id) AS HIDDEN numArticles FROM User u LEFT JOIN u.Articles a ORDER BY numArticles DESC HAVING numArticles > 10

Your result will be a collection of Users, and not an array with key 0 as User object instance and "numArticles" as the number of articles per user


# Map entities as scalars in DQL result

When hydrating to array or even a mixed result in object hydrator, previously you had the 0 index holding you entity instance.
You are now allowed to alias this, providing more flexibility for you code.
Example:

SELECT u AS user FROM User u

Will now return a collection of arrays with index "user" pointing to the User object instance.


# Performance optimizations

Thousands of lines were completely reviewed and optimized for best performance.
Removed redundancy and improved code readability made now internal Doctrine code easier to understand.
Also, Doctrine 2.2 now is around 10-15% faster than 2.1.