The Right Way to expire an intro point is to establish a new one to
replace it, publish a new descriptor that doesn't list any expiring intro
points, and *then*, once our upload attempts for the new descriptor have
ended (whether in success or failure), close the expiring intro points.

Unfortunately, we can't find out when the new descriptor has actually been
uploaded, so we'll have to settle for a five-minute timer.

There should be no significant behaviour changes due to this commit (only
a log-message change or two), despite the rather massive overhaul, so this
commit doesn't include a changes/ file.  (The commit that teaches
intro_point_should_expire_now to return non-zero gets a changes/ file,
though.)