The httpcache package is intended only to be used in private caches, so
it will cache responses marked `private` like normal. However,
imageproxy is a shared cache, so these response should not be cached
under normal circumstances. This change introduces a potentially
breaking change to start respecting the `private` cache directive in
responses.
This also adds a new `-forceCache` flag to ignore the `private` and
`no-store` directives, and cache all responses regardless.