This implements the Expiry interface so token expiry works properly
This change makes sure that this change from the swift library works
correctly with rclone's custom authenticator.
> Renew the token 60s before the expiry time
>
> The v2 and v3 auth schemes both return the expiry time of the token,
> so instead of waiting for a 401 error, renew the token 60s before this
> time.
>
> This makes transfers more efficient and also works around a bug in
> CEPH which returns 403 instead of 401 when the token expires.
>
> http://tracker.ceph.com/issues/22223
Nick Craig-Wood