Previously, we used the existence of a `prior*` field in the Resource object as an indication for whether we should consult the cache or not. However, this is prone to error, since a failed cache lookup won't set any prior fields. Therefore, we manually set `priorExpires` to 0. This in turn triggered another bug where generated wrong expiration timestamps when the server response we got was expired (or expired between sending and receiving).
This commit changes the flags so that we can now explicitly request CacheOnly/NetworkOnly (or All) loading methods, rather than the implicit Optional/Required naming scheme.
In https://github.com/mapbox/mapbox-gl-native/pull/9670, we implemented support for the Cache-Control: must-revalidate header. While we now respect this header and don't show resources that are stale and have this header set, the optional cache request never completes. This means we're also never going to try to actually get a fresh tile and just never show this tile anymore.
This commit fixes this by responding with a Not Found error when the resource is unusable (= stale and has must-revalidate set). Since we actually still have the data (but can't use it due to caching rules), we're responding with the data as well. To avoid a second cache hit, tile_loader_impl.hpp now passes on the data from the Optional to the Required request so that it can be reused when we get a 304 Not Modified response from the server.
