Extended StatalServer and StatalServerCursor to handle payloads of length 1.

Made the relevant change in PayloadTree._get_next_n_nodes_at_level_helper():

    # special case to handle a cyclic tree of length 1
    elif node.is_at_level(0) and len(node) == 1:
        if previous_node.is_at_level(level):
            result.append(node)

Those three lines were all that were required to implement the extension.

The remaining parts of this commit beautify parts of the classes that
collaborate on the feature and provide new doctests as regression.