RESTful Facets service: multi-facets (#629).

Updating multi-facets is now functional.

BREAKING CHANGE

Facets API and REST API has changed.

New class:
    de.deepamehta.plugins.facets.model.FacetValues

{{{
void updateFacets(long topicId, String facetTypeUri, FacetValues facetValues, ClientState clientState)
PUT /facet/multi/{facet_type_uri}/topic/{id}
{JSON object in message body}

void updateFacets(DeepaMehtaObject object, String facetTypeUri, FacetValues facetValues,
                                                       ClientState clientState, Directives directives)
}}}

The JSON object in the message body is a composite value with exactly one entry. Key is the child type URI, value is an array. Syntax is exactly like in an update request. All types of array elements are supported: topic values (compact format, strings), entire topic models, or topic references (by ID or by URI).

Example (topic references):

PUT /facet/multi/ka2.criteria.zielgruppe.facet/topic/3575
{
    "ka2.criteria.zielgruppe": ["ref_uri:ka2.category.erwachsene", "ref_uri:ka2.category.jugendliche"]
}

See #629