util/disk_cache: do crc32 check on compressed data for ZSTD

This will be faster and avoids checking for errors with the compression implementation which we shouldn't need to do. Instead we trust the compression library does the correct thing and simply error check the data loaded from disk. For reference this is the way the tool Fossilize does its crc checks on compressed archives.

For now this is only done for ZSTD as our zlib implementation doesn't always fully compress the cache items before writting parts of them to disk.