Task scheduler: Optimize parallel loop over lists

The goal is to address performance regression when going from
few threads to 10s of threads. On a systems with more than 32
CPU threads the benefit of threaded loop was actually harmful.

There are following tweaks now:

- The chunk size is adaptive for the number of threads, which
  minimizes scheduling overhead.

- The number of tasks is adaptive to the list size and chunk
  size.

Here comes performance comparison on the production shot:

 Number of threads        DEG time before        DEG time after
       44                     0.09                   0.02
       32                     0.055                  0.025
       16                     0.025                  0.025
       8                      0.035                  0.033

0 files changed, 0 insertions, 0 deletions