Faster hashing - proposal for next major version

[zeroby0]

People who process more than a few hundred images would see major speedups in start time if we switch to a faster non-cryptographic hash function. We can also try to read only the first few hundred kilobytes instead of reading the entire image.

However, this would change the hash, so should probably be released behind a config flag or in a major version.

Suggestion:

• https://xxhash.com/