FacetResultCollector should collect nodes by id, instead of by path

instead of collecting nodes based on their paths as the identifier, it is better that I return nodes based on their id, because later in the chain, the paths are converted to node ids, where i might return id's in the first place.

This implementation needs some changing in the ServicingNodeIndexer which does not need to store the hippo:path anymore, the FacetResultCollector returning ids, and some code after it, which expects paths