Properly wakeup ClassMetadata

ping @Ocramius 

Resource metadata holds reference to a classmetadata. However, ClassMetadata reflection fields (reflFields) are not serialized, which means that if we use APC to cache ZfrRest resource metadata, the ClassMetadata from Doctrine is not properly wake up and it crashes.

This PR fixes that by regenerating missing fields. I know it introduces a dependency to Doctrine ORM but this seems like the lightest way.