Display a warning when Hazelcast configuration does not contain Jetty session serializer

[olamy]

It's possible to configure Hazelcast Session Manager with a single hazelcast.xml file but the Jetty session manager needs to have the serializer configured as is:

  <serialization>
    <serializers>
      <serializer type-class="org.eclipse.jetty.server.session.SessionData"
                  class-name="org.eclipse.jetty.hazelcast.session.SessionDataSerializer" />
    </serializers>
  </serialization> 

If it's not configured we should display a warning in the logs telling the user the Hazelcast Jetty session integration will not work