DATAJPA-466 - Add support for lazy loading configuration via JPA 2.1 fetch-/loadgraph.

[thomasdarimont]

We now support loadgraph / fetchgraph QueryHints on repository query methods, which are applied when a JPA 2.1 capable JPA implementation is used, otherwise they are ignored.

FetchGraphs / LoadGraphs can now be defined on the Entity via the @NamedEntityGraphs annotation.

@Entity
@QueryEntity
@NamedEntityGraphs(@NamedEntityGraph(name = "GroupInfo.members", attributeNodes = @NamedAttributeNode("members")))
public class GroupInfo {
...
@ManyToMany List<GroupMember> members = new ArrayList<GroupMember>(); //default fetch mode is "lazy".
}

The entity graph "GroupInfo.members" overwrites the fetch-mode of the members collection to be "eager".

The fetch graph to be used can now configured on a repository query method.

@Repository
public interface GroupRepository extends CrudRepository<GroupInfo, String> {

    @QueryHints(@QueryHint(name = "javax.persistence.fetchgraph", value = "GroupInfo.members"))
    GroupInfo getByGroupName(String name);
}

[thomasdarimont]

Since we use the JPA 2.0 API by default it is difficult to test support for JPA 2.1 features.
We should change our test-support to be able to test support for 2.1 features as well.

I tested this with an external project.

[thomasdarimont]

We should probably make those constants public to be able to use them in repository definitions.

[odrotbohm]

As per our discussion this morning:

• add a dedicated @EntityGraph annotation that takes a name and an enum to distinguish between load and fetch graphs. This should internally be translated into the appropriate hint. What's the better default for load vs. fetch?
• ideally the annotation evaluation is done inside JpaQueryMethod so that we're able to pull up the generic parts into Spring Data Commons' QueryMethod eventually.
• we should also detect and reject the declaration of entity graphs to be used if we're not running on a JPA 2.1 persistent provider (that should also make it easier to test the inability of that stuff on JPA 2.0)