plugin XEP-0384, xml_tools: avoid `getItems` timeout + fix empty node crash + parsing: - use `max_items` in `getItems` calls for bundles, as otherwise some pubsub service may return full nodes, which may be huge is `max_items=1` is not set on the node, possibly resulting in timeouts. - the plugin was crashing when TWOMEMO devices list node has no items at all. This is not the case anymore. - a naive parsing method has been implemented in `xml_tools` to replace the serialisation/deserialisation method. This should be more efficient and will avoid annoying `ns0:` prefixes in XML logs.