PARALLEL_LAUNCH_LAST_3_LEVELS

- launch a single parallel kernel for processing last three plys. Which
means all leaf nodes at depth 5 are computed by a single kernel launch.
The default/previous approach was to launch last two plys (all leaf
nodes at depth 4 were computed by a single launch).
- doesn't improve performance for 'normal' board positions and even
hurts good positions (positions with high branching factor).
- however this gives a great boost in performance for positions with low
branching factor.
- This is mostly in preparation of adding hash table support. As hash
table hits will reduce no of active threads, launching 3 plys should
keep the occupancy high enough.
- Hopefully the last preparation change before I add hash table support
:-)