Fix Model synopsis response col

If response had to be converted to an Enum, then it's name was not
getting printed out.
More optimizations

DRF drop CollectPreds pass; predictions available from prior pass in the
histograms; pass preds thru the Split object.  Merge 2 passes.
GBM on 2 classes only builds 1 tree now.
Throttle scoring during building for fast-building but slow-scoring
models.
Lots of debug prints left in code