Blender Git Loki

Revert "Task scheduler: Avoid mutex lock in number manipulation functions"

Appears mutex was guarateeing number of tasks is not modified at moments
when it's not expected. Removing those mutexes resulted in some hard-to-catch
locks where worker thread were waiting for work by all the tasks were already
done.

This reverts commit a1d8fe052ccd8945f14be5a50bd5af581b89c643.