MINOR: cache: Change hash function in default normalizer used in case of "vary"

MINOR: cache: Change hash function in default normalizer used in case of "vary"

When building the secondary signature for cache entries when vary is
enabled, the referer part of the signature was a simple crc32 of the
first referer header.
This patch changes it to a 64bits hash based of xxhash algorithm with a
random seed built during init. This will prevent "malicious" hash
collisions between entries of the cache.