Update nodes after a do_move()

And also store the node counter in Position and not in Thread.
This will allow to properly count nodes also in sub trees with
SMP active.

This requires a surprisingly high number of changes
in a lot of places to make it work properly.

No functional change but node count changed for obvious reasons.

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

src/benchmark.cpp

@@ -161,7 +161,7 @@ void benchmark(const string& commandLine) {
       } else {
           if (!think(pos, false, false, dummy, dummy, 0, maxDepth, maxNodes, secsPerPos, moves))
               break;
-          totalNodes += nodes_searched();
+          totalNodes += pos.nodes_searched();
       }
   }