We currently store response headers in the cache entry file, before the
response body. When we implement cache revalidation, we will need to
update the stored response headers with whatever headers are received
in a 304 response. It's not unlikely that those headers will have a size
that differs from the stored headers. We would then have to rewrite the
entire response body after the new headers.
Instead of dealing with those inefficiencies, let's instead store the
response headers in the cache index. This will allow us to update the
headers with a simple SQL query.