Correctly handle count parameter @ HistoryDict.save_node_value

The current implementation is actually a no-op since the data is truncated
correctly but not assigned back to self._datachanges.
Pop the first value instead since this function is called for every
datachange which means that not more than one value will every be removed
in a single call.

opcua/server/history.py

@@ -94,7 +94,7 @@ class HistoryDict(HistoryStorageInterface):
             while now - data[0].ServerTimestamp > period:
                 data.pop(0)
         if count and len(data) > count:
-            data = data[-count:]
+            data.pop(0)
 
     def read_node_history(self, node_id, start, end, nb_values):
         cont = None