Changed the lazy_translation_getter to use prefetch_related

Using the equivalent of instance.translations.all() instead of TranslationModel.object.filter(master=instance) allows to use the Django's prefetch_related.
As opposed to the language(), it will behave in the same way as if there was no prefetch_related (ie, using the current language, then the default, then any other) while language() just skips anything that isn't translated.