Age calculations:
	There will be an Age: header.

	In order to know if a cached entry is fresh, a cache needs to
	know if its age exceeds its freshness lifetime.  The latter can be
	reliably calculated as Expires: - Date:, or from Cache-control:
	max-age=NNN (which takes priority).
	An entry's age can be calculated in two independent ways:
		now - Date:
	if the clocks are reasonably well synchronized
		Sum of "time resident in cache" for all caches involved,
			using the Age: header
	if all of the caches support Age:
	Given that we have two independent ways to compute the age,
	we can combine these as
		age = max(now - Date:, Age:)
	and as long as we have either synchronized clocks or all-HTTP/1.1
	paths, one gets a reliable (conservative) result.  The purpose
	of the Age: header is to pass this value along to the next
	recipient cache.
	Note that this correction can (but is not required to) be
	applied at each HTTP/1.1 cache along the path, so that if
	there is an HTTP/1.0 cache in the path, the correct age is
	computed as long as the receiving cache's clock is in sync.
	We don't need end-to-end clock synchronization (although it is
	good to have), and there is no explicit clock synchronization