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/cluster.h

@@ -69,6 +69,8 @@ inline bool is_root() { return rank() == 0; }
 void save(Thread* thread, TTEntry* tte,
           Key k, Value v, Bound b, Depth d, Move m, Value ev);
 void reduce_moves(MoveInfo& mi);
+void sync_start();
+void sync_stop();
 
 #else
 
@@ -86,6 +88,8 @@ inline void save(Thread* thread, TTEntry* tte,
   tte->save(k, v, b, d, m, ev);
 }
 inline void reduce_moves(MoveInfo&) { }
+inline void sync_start() { }
+inline void sync_stop() { }
 
 #endif /* USE_MPI */