http://idk.i2p/javadoc-i2p/org/klomp/snark/web/FetchAndAdd.html
To track _actual_ transfer if the output file could already exist,
the listener should keep its own counter,
or subtract the initial alreadyTransferred value.
And watch out for alreadyTransferred resetting if a resume failed... currentWrite - since last call to the listener bytesTransferred - includes headers, retries, redirects, discarded partial downloads, ... bytesRemaining - on this attempt only, currentWrite already...