get_listener() is allowed to return either, and the Tor provider is currently
simple enough to not really need more than a basic descriptor, but have it
return a full Endpoint for use as an example of what I2P can do later.
This delegates the construction of the server Endpoint object to the i2p/tor
Provider, which can use the i2p/tor section of the config file to add options
which would be awkward to express as text in an endpoint descriptor string.
refs ticket:2889 (but note this merely makes room for a function to be
written that can process I2CP options, it does not actually handle such
options, so it does not close this ticket yet)
This uses a unix-domain control port, and includes test coverage.
create_onion() displays pacifier messages, since the allocate-onion step
takes around 35 seconds
