Taxonomy MixinClassificationDaoPlugin: store parent keys on documents

Picked taxonomy categories (a.k.a. terms) are stored on document level in a multiple property, by default hippotaxonomy:keys.

The parent key information is not stored on documents, unless the new button 15.2 "Select term and ancestors". If not stored, document's cannot be found for higher level categories, in faceted navigation or other searches (in the delivery tier).

A solution for this can be to save all keys always, e.g. in hippotaxonomy:allkeys hippotaxonomy:keyswithancestors, so separately from which categories are selected in the picker.

Also, the taxonomy tree that a selected category is part of, is not saved on documents. This can lead to problems if two categories belonging to different trees have the same key probably not needed now