MEDIUM: cache/compression: Add a way to safely combined compression and cache

This is only true for HTX proxies. On legacy HTTP proxy, if the compression and
the cache are both enabled, an error during HAProxy startup is triggered.

With the HTX, now you can use both in any order. If the compression is defined
before the cache, then the responses will be stored compressed. If the
compression is defined after the cache, then the responses will be stored
uncompressed. So in the last case, when a response is served from the cache, it
will compressed too like any response.
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