Merge commit 'c8213ba0d047569141ed58f5eb86579d976b5614' into cluster

As part of this work, we needed to rename the namespace Stockfish::Cluster
to Stockfish::Distributed, as Cluster is now a type name.

src/thread.cpp

@@ -296,7 +296,7 @@ void ThreadPool::start_thinking(const OptionsMap&  options,
     for (auto&& th : threads)
         th->wait_for_search_finished();
 
-    Cluster::signals_init();
+    Distributed::signals_init();
 
     main_thread()->start_searching();
 }