title: opcua - not getting all values in the queue after setting sample rate
body: <p>This is my code in python:</p>
  <pre><code>client = Client('opc.tcp://DESKTOP-D1B43P9.mshome.net:53530/OPCUA/SimulationServer')
  client.connect()
  counter = client.get_node(&quot;ns=3;i=1001&quot;)
  handler = SubHandler()
  sub = client.create_subscription(10000, handler)
  handle = sub.subscribe_data_change(counter, queuesize=2)
  sub.modify_monitored_item(handle, 500)
  </code></pre>
  <p>If I don't change the sampling rate (remove the last line), I get two values (datachange_notification is called twice in a row) every 10 seconds.
  But if I do change the sampling rate, it's called only once.</p>