Close the messaging client on a data node when closing

Just setting it to nil was leaking resources

server.go

@@ -262,8 +262,10 @@ func (s *Server) close() error {
 		s.done = nil
 	}
 
-	// Remove client.
+	if s.client != nil {
+		s.client.Close()
 		s.client = nil
+	}
 
 	// Close metastore.
 	_ = s.meta.close()