Improve load from storage & process

[claudiu-cristea]

Cloned from ec-europa/rdf_entity#19

Next methods should be improved:

• \Drupal\rdf_entity\Entity\RdfEntitySparqlStorage::loadFromStorage(): Address the @todos. Note that we can use ::idExists() somehow to avoid loading entities from all the graphs. But this still needs profiling because ::idExists() is making additional queries.
• \Drupal\rdf_entity\Entity\RdfEntitySparqlStorage::processGraphResults() needs some love to make it more readable. Probably it needs a helper method.
• Review also allhelper methods used by these 2.