CobraConnection: sets a unique id field for all messages sent to [cobra](https://github.com/machinezone/cobra).

CobraConnection: sets a counter as a field for each event published.

ws/ixcobra/IXCobraConnection.h

@@ -168,6 +168,9 @@ namespace ix
 
         // Cap the queue size (100 elems so far -> ~100k)
         static constexpr size_t kQueueMaxSize = 256;
+
+        // Each pdu sent should have an incremental unique id
+        std::atomic<uint64_t> _id;
     };
 
 } // namespace ix