Don't wake up threads at the beginning of the search

But only when needed, after a split point. This behaviour
does not apply when useSleepingThreads is false, becuase
in this case threads are not woken up at split points so
must be already running.

No functional change.

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

src/search.cpp

@@ -307,11 +307,7 @@ void Search::think() {
           << endl;
   }
 
-  for (int i = 0; i < Threads.size(); i++)
-  {
-      Threads[i].maxPly = 0;
-      Threads[i].wake_up();
-  }
+  Threads.set_size(Options["Threads"]);
 
   // Set best timer interval to avoid lagging under time pressure. Timer is
   // used to check for remaining available thinking time.