The frame marker system supports one to map the reference frame
index into the natural order. It allows direct checking on the
efficacy of the reference frames given their relative locations
with respect to the current coding frame.

This commit uses such property to filter out reference frames
less likely to contribute coding gains from the rate-distortion
optimization process. For example, it takes out the check on
last2 / 3 frames, when their actual location is further away
from the golden frame.

The AWCY results show 0.6% performance regression. The encoding
speed gets doubled.

To use the speed up, one needs to turn on frame-marker experiment
before we turn it on by default, and enable selective_ref_frame
entry in the speed feature.

Change-Id: Ifb03ed90acd980bbc7ff1c2e17982e21e68d2588