NotImplemented Exception when using Server with cache file

[fixing-it]

Describe the bug
NotImplementedError exception is raised after second start when using Server with cache file

To Reproduce
Steps to reproduce the behavior incl code.
self.server = Server(shelf_file=cache_path)

Expected behavior
Exception should not be raised, since the load_aspace_shelf function from address_space.py seems to be implemented

def load_aspace_shelf(self, path):
    """
    Load the standard address space nodes from a python shelve via LazyLoadingDict as needed.
    The dump() method can no longer be used if the address space is being loaded from a shelf

    Note: Intended for slow devices, such as Raspberry Pi, to greatly improve start up time
    """
    raise NotImplementedError

Version
Python-Version:3.9
python-opcua Version (e.g. master branch, 0.9):master

[swamper123]

What i still know is, that there was an attempt to realise that feature to load stuff faster. Most of the lines of load_aspace_shelf haven't been touched since then.

Not sure if this is still on a ToDo List @oroulet ? Or is that an abandoned feature?