Added stub for server-to-server messages

source/libdnet/protobuf/server.proto

@@ -15,6 +15,7 @@ message ServerMessage
         LIST_USERS = 1;
         GET_SERVER_PROPS = 2;
         GET_SERVER_PROP = 3;
+        S2S_MESSAGE = 4;
     }
 
     // The type of the server command
@@ -22,4 +23,15 @@ message ServerMessage
 
     // Additional data (dependent on command)
     bytes additionalData = 2;
+}
+
+// STS
+//
+// These represent server-to-server messages
+// for linking, delinking, transferring data
+// for users, channels, messages and other
+// network-wide information
+message STSMessage
+{
+    
 }