A speedup feature that enforces the maximum number of reference frames
to evaluate for each arbitrary frame to be 6, as opposed to the maximum
syntax-allowed number of reference frames which is 7, through the
following rules:

(1) When all the possible reference frames are availble, we reduce the
    number of reference frames (7) by 1 by removing one reference
    frame;
(2) Always retain GOLDEN_FARME/ALTEF_FRAME;
(3) Check the earliest 2 remaining reference frames, and there are two
    options to remove one reference frame:
    (a) Remove the one with the lower quality factor; Otherwise if
    both frames have been coded at the same quality level, remove the
    earliest reference frame;
    (b) Always remove the earliest reference frame.
    Currently set option (a).

This patch has demonstrated an encoder speedup of ~8.5%. It is under
speed2 with "selective_ref_frame >= 2". (May be considered to move
to speed1 later once the coding performance impact is evaluated
further.)

Using the following configure setups, the coding performance has been
dropped on Google test sets (50 frames) in BDRate by ~0.2% for lowres
and by ~0.1% for midres:
--enable-experimental --disable-convolve-round --disable-ext-partition
--disable-ext-partition-types --disable-txk-sel --disable-txm

Change-Id: I84317bae00bfd90b2c3d301858a849f441974e6f