Use better hash function and approach for the Cache key.

The cache hash was calculated using CRC64, which in principle makes it possible to poison the cache. If you know the SELECT you want to poison, you can make another SELECT result in the same CRC, by interspersing the SQL with a suitable pattern of spaces and tabs. This is of concern only if users=mixed is used, which should only be used in a high-trust environment. The CRC64 is now changed to XXH3-128, which removes this possibility.