Introduce and use wait_for_search_finished()

Helper function that allows us to simplify
the code.

No functional change.

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

src/benchmark.cpp

@@ -118,7 +118,7 @@ void benchmark(int argc, char* argv[]) {
       }
       else
       {
-          Threads.start_thinking(pos, limits);
+          Threads.start_searching(pos, limits);
           nodes += Search::RootPosition.nodes_searched();
       }
   }