Cleans up the experiment. Actually uses reduced counts for backward
updates, and reduced number of probabilities in the context.

No change in bitstream when the experiment is on.

Between expt on and off:
derfraw300 is down only -0.062% (which is better than when expts
were run previously).

Change-Id: I55285a049a0c22810bdb42914212ab5a4f8521b5