[PATCH 1 of 2] Cache: remove unused valid_msec fields

[Piotr Sikora]

Hey Maxim,

> I would rather not, for two reasons:
>
> - I would actually like to see support for subsecond cache
>   validity times added eventually (unlikely to happen in the near
>   future, but still).

That's valid reason, but it's going to be of rather limited use,
because neither "Expires" nor "Cache-Control" can request such
validity.

Also, this field has been there since the introduction of cache over 5
years ago, just wasting I/O, so I'd argue that there is no reason to
keep it, unless it's being used (which is not). It can always be added
later on, when it's needed.

> - This implies cache file on-disk format change and invalidation
>   of all previously cached files, so shouldn't be done without
>   some serious enough reason.

Trust me, I know that ;) That's why it's part of a patchset that adds
status field to the on-disk format (in the exactly same spot,
nonetheless).

Also, I'm going to do another on-disk format change during next
release cycle (for the Age and Expires headers, that we've talked
about), so you might postpone committing this if you're worried about
too much bumps, but we're going to invalidate all previously cached
files one way or another.

Best regards,
Piotr Sikora