Migrate from sha1 to xxhash for deduplication methods

[Hynek Kydlíček]

Problem

We currently use first x bytes sha1 for hashing, which is a waste of resources.

• we don't need cryptographic guarantees
• we only take first x bytes

Instead we should use non-cryptographic hash function, which can be computed significantly faster.
This should speed significantly the first phase of deduplication process