This commit enables asymptotic closed-loop encoding decision for
the key frame and alternate reference frame. It follows the regular
rate control scheme, but leaves out additional iteration on the
updated frame level probability model. It is enabled for speed 0.

The compression performance is improved:

lowres 0.2%
midres 0.35%
hdres  0.4%

Change-Id: I905ffa057c9a1ef2e90ef87c9723a6cf7dbe67cb