Only use 127.0.0.1 as local address

[duck]

For the anonymous network use case (such as ​I2P), we want to only use 127.0.0.1 as loopback address. Right now Tahoe discovers all local addresses through various strategies and discloses them to (atleast) the introducer.

For I2P we have introduced the configuration option anonymize_local_addresses (which we consider renaming to tub.anonymize) to disable this lookup.

Example of configuration in tahoe.cfg:

    [node]
    ...
    anonymize_local_addresses = true

Snippit showing how this is used in node.py:

    if self.get_config("node", "anonymize_local_addresses", False, boolean=True):
        d.addCallback(lambda res: ['127.0.0.1'])
    else:
        d.addCallback(lambda res: iputil.get_local_addresses_async())
    d.addCallback(self._setup_tub)