PATCH: stale-while-revalidate support

[Nils Goroll]

Federico,

after our irc discussion, I could not come up with a more precise suggestion on
how to implement s-i-e due to the problems I have just posted in "restarting for
bad synchronous responses", so unless there is or will be a way to achieve a
restart (not a retry) for the "error response case", I agree now that we can't
get s-i-e support.

So what remains is that I think we should support the case where the object we
receive has age > max-age and put it into the cache with 0 ttl and appropriate
grace. The rfc is clear about this (second example).

We might need to touch more code to get ttl == 0 working.

Thanks, Nils