I am reviving temp-tbb-task-scheduler. There were too many changes in master to do a clean rebase/merge (without knowing the exact state of master and that branch). So my attempt is to cherry pick the commits from that branch one by one. To not diverse too much from master we could decide to add these as early as possible to master.

This patch contains the first 2 commits

1. Tasks: move priority from task to task pool rBf7c18df4f599: Tasks: move priority from task to task pool
2. Tasks: split task.c into task_pool.cc and task_iterator.c rB4ada1d267749: Tasks: split task.c into task_pool.cc and task_iterator.c

After this patch we can continue with the map-reduce as that needs some additional development.