vp9_ethread: add parallel loopfilter

vp9_ethread: add parallel loopfilter

1. Added row-based loopfilter in encoder;
2. Moved common multi-threaded loopfilter functions from decoder
   to common;
3. Merged multi-threaded loopfilter code, and made encoder/
   decoder call same function to reduce code duplication.

Encoder tests showed that 1% - 2% speedup was seen for good-quality
2-pass mode(at speed 3); 1% - 3% speedup using 2 threads and 4% - 6%
speedup using 4 threads were seen for real-time mode(at speed 7).

Change-Id: I8a4ac51c2ad9bab9fa7b864e90743931c53ec1c4