Cache: support for stale-while-revalidate and stale-if-error. Previously, there was no way to enable the proxy_cache_use_stale behavior by reading the backend response. Now, stale-while-revalidate and stale-if-error Cache-Control extensions (RFC 5861) are supported. They specify, how long a stale response can be used when a cache entry is being updated, or in case of an error.

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