Reduce history length requirement about minigo HOT 2 OPEN

One concrete idea: instead of selecting 2% flat from the last 50 generations, select 4%->0% over the last 50 generations, with some sort of exponentially decaying curve, and also make this parameter configurable. Early on, we might want to have 10% -> 0% over the last ~10 generations of data, but later on we might want to flatten that curve to select 2% -> 0% over the last 100 generations.

from minigo.

I'm closing this item and adding a note to the open ideas issue

from minigo.