Implement proper stop signalling from root node

Previous behavior was to wait on all nodes to finish their search on their own TM and aggregate to root node via a blocking MPI_Allreduce call. This seems to be problematic.

In this commit a proper non-blocking signalling barrier was implemented to use TM from root node to control the cluster search, and disable TM on all non-root nodes.

Also includes some cosmetic fix to the nodes/NPS display.

src/thread.cpp

@@ -163,6 +163,8 @@ void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
   main()->wait_for_search_finished();
 
   stopOnPonderhit = stop = false;
+  Cluster::sync_start();
+
   ponder = ponderMode;
   Search::Limits = limits;
   Search::RootMoves rootMoves;