Allow split point master to sleep (take 2)

Let to sleep even split point master, it will be waken
up by its slaves when they return from the search.

This time let it be enabled by an UCI option, so
people is free to test it on their Hyper Thread box.

Option is disabled by default.

No functional change.

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

src/thread.h

@@ -55,6 +55,7 @@ struct SplitPoint {
   bool pvNode, mateThreat;
   Value beta;
   int ply;
+  int master;
   Move threatMove;
   SearchStack sstack[MAX_THREADS][PLY_MAX_PLUS_2];