req.max_stale_while_revalidate
RTIME, can be read and set, but not unset.
Available inall subroutines.
Same as req.max_stale_if_error except controls the maximum stale age for
stale while revalidate functionality.
If an object's TTL has expired and it is eligible to be served stale and to be served stale while revalidating, stale while revalidate takes precedence.