Revert "Revert "Loopfilter MultiThread Optimization""

This reverts commit 753fd86e86ac727dccac88376260b8f54502f2a3.

This also has the fix for the DoS reported in bug 1558.

BUG=webm:1558

Change-Id: I65ea84e0c11d6bd40d8cb0587dfe934b3ac11dce
9 files changed