Revise CachePolicy based on what we've learned.

[toddmgreer]

Commit Message:
Revise CachePolicy interface based on experience using a forked version.
Additional Description:

• CachePolicy can now be stored in FilterState
• CacheEntryUsability:
  • Added ttl for use in cache-status header
  • Added op==, op!= for use in tests
• requestCacheable:
  • Now returns enum, so it can say it's ok to serve from cache but not insert into cahce
  • Renamed to requestCacheability to reflect not returning a bool
• responseCacheable:
  • Now returns enum, so it can say to mark a cache key as uncacheable to enable later optimizations
  • Renamed to responseCacheability to reflect not returning a bool
  • Removed VaryHeader parameter--CacheFilter should only call responseCacheable if the Vary headers are ok
• createCacheKey: renamed to cacheKey, to avoid implying that the key must literally be created here
• Removed *CacheControl parameters--not always needed; CachePolicy implementations can make then if needed
• Removed setCallbacks--CachePolicy implementations that need FilterState should take it as a ctor parameter
  Risk Level: None--not yet used
  Testing: n/a--it's just an interface
  Docs Changes: n/a--not yet used
  Release Notes: n/a--not yet used
  Platform Specific Features: n/a

[toddmgreer]

/assign @mkbehr

[mkbehr]

Can you add a summary to the commit description of what's changing and why?

[mkbehr]

Remaining relative to when?

[toddmgreer]

Clarified

[toddmgreer]

Can you add a summary to the commit description of what's changing and why?

I don't know how to do that. A maintainer with merge authority will eventually squash all commits into one, and I don't know how that commit gets its description. I guess we can bring this up with the maintainer once that review gets going.

[jmarantz]

Maintainers usually cut and paste from the Deescription to populate the message for the squashed commit.

[mkbehr]

Can you add a summary to the commit description of what's changing and why?

I don't know how to do that. A maintainer with merge authority will eventually squash all commits into one, and I don't know how that commit gets its description. I guess we can bring this up with the maintainer once that review gets going.

There's a comment at the top of the pull request that got populated with a template, and IME that's where the description comes from. I think github will let you edit the comment.

This looks good to me so far but I want to make sure I understand everything that's different from our internal changes (e.g. removing the callbacks) before I hit approve, so the summary will help that.

[toddmgreer]

Can you add a summary to the commit description of what's changing and why?
There's a comment at the top of the pull request that got populated with a template, and IME that's where the description comes from. I think github will let you edit the comment.

Thanks! Updated.

[toddmgreer]

@jmarantz this is ready for your review

[jmarantz]

one minor nit and I think I can merge this, as it's just an extension.

[jmarantz]

I think Raven asked this already but I missed the answer, but how does a no-store request get served from cache if the response cannot be stored? Maybe you could add that to the comment here?

[toddmgreer]

Expanded explanation. Is that sufficiently clear?

[jmarantz]

Thanks!

[jmarantz]

Needs format fix

[jmarantz]

/wait

[toddmgreer]

Fixed formatting/spelling.

[toddmgreer]

/retest