avoid too frequent SmartShunt data copies (#596)

currently the whole SmartShunt data structure is copied to the
BatteryStats instance in every loop, even though the data cannot
possibly have changed. this is quite an expensive task to do in every
loop. this change tracks the last update timestamp and only does the
copy operation if an actual updated data structure was received from
the smart shunt.

include/VictronSmartShunt.h

@@ -11,6 +11,7 @@ class VictronSmartShunt : public BatteryProvider {
     std::shared_ptr<BatteryStats> getStats() const final { return _stats; }
 
 private:
+    uint32_t _lastUpdate = 0;
     std::shared_ptr<VictronSmartShuntStats> _stats =
         std::make_shared<VictronSmartShuntStats>();
 };

src/VictronSmartShunt.cpp

@@ -28,5 +28,9 @@ bool VictronSmartShunt::init(bool verboseLogging)
 void VictronSmartShunt::loop()
 {
     VeDirectShunt.loop();
+
+    if (VeDirectShunt.getLastUpdate() <= _lastUpdate) { return; }
+
     _stats->updateFrom(VeDirectShunt.veFrame);
+    _lastUpdate = VeDirectShunt.getLastUpdate();
 }