DocValues can be used to hold the byte encoding all facet ords for
the document, instead of payloads. I made a hacked up approximation
of in-RAM DV (see CachedCountingFacetsCollector in the patch) and the
gains were somewhat surprisingly large:
I didn't think payloads were THAT slow; I think it must be the advance
We need to separately test on-disk DV to make sure it's at least
on-par with payloads (but hopefully faster) and if so ... we should
cutover facets to using DV.