Can't use DrillSideways with both RangeFacetRequest and non-RangeFacetRequest

I tried to combine these two and there were several issues:

• It's ... really tricky to manage the two different
  FacetAccumulators across that N FacetCollectors that DrillSideways
  creates ... to fix this I added a new MultiFacetsAccumulator that
  switches for you.

• There was still one place in DS/DDQ that wasn't properly handling
  a non-Term drill-down.

• There was a bug in the "collector method" for DrillSideways
  whereby if a given segment had no hits, it was skipped, which is
  incorrect because it must still be visited to tally up the
  sideways counts.

• Separately I noticed that DrillSideways was doing too much work:
  it would count up drill-down counts and drill-sideways counts
  against the same dim (but then discard the drill-down counts in
  the end).