Multithreaded encoder, late sync loopfilter

Sync with loopfilter thread just at the beginning of next frame encoding.
This returns control to application faster and allows a better multicore scaling.
When PSNR packets are generated the final filtered frame is needed imediatly
so we cannot delay the sync.

Change-Id: I288d97b5e331d41d6f5bb49d97986fa12ac6f066