Move pawn and material tables under Thread class

This change allows to remove some quite a bit of code
and seems the natural thing to do.

Introduced file thread.cpp to move away from search.cpp a lot
of threads related stuff.

No functional change.

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

src/material.h

@@ -65,8 +65,8 @@ private:
 
 class MaterialInfoTable : public SimpleHash<MaterialInfo, MaterialTableSize> {
 public:
-  MaterialInfoTable();
   ~MaterialInfoTable();
+  void init();
   MaterialInfo* get_material_info(const Position& pos) const;
   static Phase game_phase(const Position& pos);