Blender Git Loki

Attempt to address nearly-starving cases.

Idea here is to reduce number of threads a pool is allowed to work on,
in case it does not get tasks quickly enough.

This does not seem to be really great result (have to only do the checks
once every 200 tasks pushed to avoid too much overhead), but I cannot
reproduce that nearly-starving case here so far. @sergey, curious if it
gives any difference on your 12cores with 14_03_G?