Date and cache validations

Hi fellow Squid hackers.

As said earlier the Date header should be updated on cache validations
in order to keep freshness controls operating properly, and I need some
input on which approach to select

a) We could track this somehow in the cache index

b) Rewrite the whole object only to get the new Date header in there

c) Figure out some way of updating the Date header in the disk object
without rewriting the whole object from scratch

The code as is today attempts to at least update the in-memory headers,
but even this seems to fail.. but regardless the update should be
permanent I think..

--
Henrik