Differential encoding of probability updates

Adds differential encoding of prob updates using a subexponential
code centered around the previous probability value.
Also searches for the most cost-effective update, and breaks
up the coefficient updates into smaller groups.

Small gain on Derf: 0.2%

Change-Id: Ie0071e3dc113e3d0d7ab95b6442bb07a89970030
10 files changed