Don't split if reduced depth is below min_split_depth

Don't split if reduced depth is below min_split_depth

It seems to increase SMP performances. To note that
this patch goes in the opposite direction of "Active
reparenting" where we try to reparent an idle slave
as soon as possible. Instead here we prefer to keep
it idle instead of splitting on a shallow / near the
leaves node.

After 11550 games on a QUAD (4 threads) at 15"+0.05
Mod vs Orig 1972 - 1752 - 7826 ELO +6 (+-3.6)

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>