Enhancement: Use optimized library for hash calculation of file chunks

We've switched the library used to calculate the hashes of file chunks, which
are used for deduplication, to the optimized Minio SHA-256 implementation.

Depending on the CPU it improves the hashing throughput by 10-30%. Modern x86
CPUs with the SHA Extension should be about two to three times faster.

https://github.com/restic/restic/issues/551
https://github.com/restic/restic/pull/2709
