director: Avoid USER loops with >1s ring latency also with old directors

director: Avoid USER loops with >1s ring latency also with old directors

Do this by ignoring USER refreshes that were already updated recently.
The "recently" is calculated by director_user_expire/4 seconds ago, but
with an upper limit of 15 secs. This means that the USER loops can now
only exist if the director ring latency is above 15 seconds. (Once all
directors in the ring are running the new version, there's no looping
at any latency.)