Move GetChange#cache to a better-named Response static factory

The old method wasn't telling the client to not cache per se, but
instead to use the cached version after revalidation. "Cache" was
sufficiently generic to be confusing.

Move this to a factory method in Response, and give it the slightly
more descriptive name withMustRevalidate.

Change-Id: Ic0bf486637f4542de3a2e6088eff1fee2d26c0d7