Per our design, every connection starts out unauthenticated, and
needs one authenticate command to become authenticated.
Right now the only authentication type is "This is a unix named
socket where everybody who can connect has permission."
Ordinarily you can cancel a future just by dropping it, but we'll
want the ability to cancel futures that we no longer own (because we
gave them to a `FuturesUnordered`).
Nick Mathewson