Use subclassing to inherit from `PostgresNode` and create a hierarchy
containing `AccessNode` and `DataNode` to simplify creating tests with
multiple nodes.
Also, two new functions are added:
`TimescaleNode::create`: Creates the new node by calling
`get_new_node`, `init` and `start` in that order.
`AccessNode::add_data_node`: Adds a new data node to the access node.
Also rewrite the test to use the new hierarchy.
