This means you can make estimates about storage-time remaining based on the
results of the calculator endpoint and they'll at least match your local
configuration.

also, move the function to the config module.

It turns our that `pyflakes` with no arguments expects input
on stdin. In CI, this is likely connected to `/dev/null` so it
passes.

Also fix the errors that pyflakes is reporting.

It was orginally factored out since there were both `add_lease` and `renew_lease methods`.
However, since tahoe 1.16, the client no longer defines `renew_lease`, and we removed it
from our protocol in #231. Given that there is now only one caller, we can remove the
factoring.

avoids ambiguities and implementation problems of iso8601 duration strings

go back to aniso8601 dependency, since we no longer get anything distinctive
from isodate

now client and server agree that only shares with associated writes in
`tw_vectors` are considered when computing cost

The remote method does not expose the renew_leases parameter, intentionally.

Previously an active lease on *any* share would cause us to disable lease
renewal entirely - including for newly written shares.

Now we will add a lease to any changed share that needs one, either because it
is growing or because it has no unexpired lease.

Previously this only exercised creation when no shares existed.  Now it will
create some shares and then create some more.

There is no longer an extra bool argument controlling lease renewal that we
have to protect from use by clients.