Create docvalues based grouped facet collector. Currently only term based collectors have been implemented (LUCENE-3802).
Initial patch containing a implementation for bytes sorted. Random test fails for now...
I think bytes sorted is the only impl that makes since other implementations don't work with ordinals and thus would be much slower.
Updated patch. Random test passes now. Need to add some more jdocs and remove some duplicate code and then it should be ready.
Some code refactorings and added some more jdocs.
Committed to trunk.