Loopfilter Multi-Thread Optimization

Take the original loopfilter multi-thread optimization
(dafe064289a917977439ab6f4f002b9946496084) along with the fixes for bugs
1558 and 1562.

BUG=webm:1558
BUG=webm:1562

Change-Id: Ibbf6bd13f4ffff0e79184ccfd6b85a49e067a6d8
13 files changed