Material: lockless per-thread maps

Adds a good bunch of code but should be faster
and scalable because is lockless.

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

src/position.h

@@ -117,6 +117,7 @@ struct UndoInfo {
 class Position {
 
   friend class MaterialInfo;
+  friend class EndgameFunctions;
 
 public:
   // Constructors