Restore ClassMetadataInfo::$lifecycleCallbacks array structure (fixes #803)

This restores the structure before 59a67f3f29020f6dfc82b7446c6140b4010d8681 (#428) to be consistent with ORM while still ensuring that the same callback is not registered multiple times.
Merge pull request #839 from doctrine/gh-803

Restore ClassMetadataInfo::$lifecycleCallbacks array structure