[Cluster] Use a sendrecv ring instead of allgather

Using point to point instead of a collective improves performance, and might be more flexible for future improvements.
Also corrects the condition for the number elements required to fill the send buffer.

The actual Elo gains depends a bit on the setup used for testing.

8mpi x 32t yields 141 - 102 - 957 ~ 11 Elo
8mpi x 1t yields 70 +- 9 Elo.

src/thread.cpp

@@ -141,7 +141,7 @@ void ThreadPool::set(size_t requested) {
       TT.resize(Options["Hash"]);
 
       // Adjust cluster buffers
-      Cluster::ttRecvBuff_resize(requested);
+      Cluster::ttSendRecvBuff_resize(requested);
   }
 }