DDS plugin: Synchronize FillChannelContainers and DDSKeyValue updates

This was a regression after introducing external control mode in f7cdf5e.

fairmq/plugins/DDS/DDS.h

@@ -163,6 +163,10 @@ class DDS : public Plugin
 
     std::thread fHeartbeatThread;
     std::chrono::milliseconds fHeartbeatInterval;
+
+    bool fUpdatesAllowed;
+    std::mutex fUpdateMutex;
+    std::condition_variable fUpdateCondition;
 };
 
 Plugin::ProgOptions DDSProgramOptions()