Allow split point master to sleep

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

With this patch we get maximum HT speedup

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];