Investigate alternate hashing algorithms

[droyad]

Also, it would be interesting to use the xxHash algorithm instead of SHA1 (SHA1 has an average speed of 0.28 GB/s, while xxHash has 5.4 GB/s).

https://cyan4973.github.io/xxHash/

Originally posted by @arabasso in #21 (comment)

For Octopus we would need to figure out how we get the two ends to agree on the algorithm to be used.

[GrzegorzBlok]

@droyad xxHash64 is a default algorithm instead of SHA1 in a library that I derived from Octodiff:
https://github.com/GrzegorzBlok/FastRsyncNet
It works perfectly fine - it gives you significant faster calculations and smaller signature size.

[droyad]

@GrzegorzBlok Cool. The blocker for Octopus is negotiating between the two endpoints since either end could be running an older version of Octodiff.