Allow checking zeromv mode in vp9_pick_inter_mode

This improves the compression performance of speed -5 by 0.6%. The
speed impact is less than 1%.

Change-Id: Ie77daa561976dfc8b479061e1221bdf428eb0c3b
vp9_ethread: allocate frame contexts outside VP9_COMMON struct

This patch allocated frame contexts outside VP9_COMMON. This allows
multiple threads to share the same copy of frame contexts, and
reduces the overhead. It also guarantees the correct update of
these contexts during bitstream packing. This patch doesn't change
encoding result.

Change-Id: Ic181a2460b891d1d587278a6d02d8057b9dbd353
Merge "vp9_ethread: allocate frame contexts outside VP9_COMMON struct"
Merge "Allow checking zeromv mode in vp9_pick_inter_mode"