[dbus-writer] limit d-bus traffic

Limit d-bus traffic by only sending updates, instead
of every value.

Tested-by: used d-bus monitor and noticed significantly
reduced messages.

Change-Id: Ie677feaedab4e9ebb14950392b9588d4d82c0058
Signed-off-by: James Feist <james.feist@linux.intel.com>
diff --git a/dbus/dbuswrite.hpp b/dbus/dbuswrite.hpp
index 0bdcde9..6a70fe9 100644
--- a/dbus/dbuswrite.hpp
+++ b/dbus/dbuswrite.hpp
@@ -41,6 +41,7 @@
   private:
     std::string path;
     std::string connectionName;
+    int64_t oldValue = -1;
 };
 
 class DbusWrite : public WriteInterface
@@ -60,4 +61,5 @@
   private:
     std::string path;
     std::string connectionName;
+    int64_t oldValue = -1;
 };