Class SortedSetDocValuesFacetCounts

Compute facets counts from previously indexed SortedSetDocValuesFacetField, without require a separate taxonomy index. Faceting is a bit slower (~25%), and there is added cost on every IndexReader open to create a new SortedSetDocValuesReaderState. Furthermore, this does not support hierarchical facets; only flat (dimension + label) facets, but it uses quite a bit less RAM to do so.

NOTE: this class should be instantiated and then used from a single thread, because it holds a thread-private instance of SortedSetDocValues.

NOTE:: tie-break is by unicode sort order @lucene.experimental