addOrUpdateValue method
Add a key/value pair to the node.
Adds a K/V tuple to the node. The key must not exist prior adding.
Implementation
@override
Future<void> addOrUpdateValue(NodeValue value) async {
lastModified = DateTime.now().millisecondsSinceEpoch;
value.lastModified = lastModified;
_values[value.key] = value;
}