Improve LoadTaxonomiesService#doTask and IProductRepository

[deniger]

Performance:
ProductRepository#getLabels loads Labels from database ( if no updates found) and LoadTaxonomiesService#doTask will reimport it in database. It's useless. Samething for other taxonomies ( tags, allergens,...)

Refresh data:
if a taxonomy should be updated from the database, the idea is to delete all local data first and reimport all data in the same transaction.

Modification:
Some new method shoud be added to IProductRepository in order to perform this actions.
add new methods in IProductRepository.

• • IProductRepository.loadLabels that will call the api if required. If not required, it will return a null value
• IProductRepository.replaceLabels that will delete labels before adding all labels ( if the new labels are not null)

Maybe it will be better to avoid using null to indicate that nothing should be done