Partial merge of NEWBESTREFMV and NEW_MVREF.

This commit merges those parts of the CONFIG_NEW_MVREF
that specifically relate to choosing a better set of candidate
MV references into the NEWBESTREFMV experiment.

CONFIG_NEW_MVREF will then be used for changes relating
to the explicit coding of a cost optimized MV reference in the
bitstream as part of MV coding.

Change-Id: Ied982c0ad72093eab29e38b8cd74d5c3d7458b10