[core] fix loading of stale must-revalidate resources

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.

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