buzhash64e: with rolling hash encryption

[ThomasWaldmann]

Rather simple and very slow encryption code, needs to be tuned later IF we find this worthwhile doing.

Note:

• the encryption of a buzhash64 current sum value must only depend on the sum value and the encryption key, NOT on the position in the current stream (thus: no AES-CTR or any other cipher that does not independently encrypt blocks, this is why ECB is used here) - otherwise it would cut the chunks not only based on the content but also based on position in the stream and that would cut differently and would destroy deduplication.
• instead of encrypting just 64bit using ECB, we could collect e.g. 1024 sums, encrypt them all on in one .encrypt_many() call and then do another loop over the esums to decide where to cut. would be much less overhead than now, but needs maintaining yet another buffer efficiently.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.06%. Comparing base (a7f8070) to head (c603fb4).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/borg/chunkers/__init__.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8920      +/-   ##
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- Coverage   82.07%   82.06%   -0.01%     
==========================================
  Files          77       77              
  Lines       13490    13491       +1     
  Branches     1996     1996              
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  Hits        11072    11072              
- Misses       1753     1754       +1     
  Partials      665      665              

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[darkk]

(As a sidenote on speed). I've found it quite interesting that fast PCG RNG (numpy's default prng) is predictable, but seed reconstruction out of 64 consecutive values is a non-trivial operation — 2¼ CPU-years of computation. However, it's still just a €108, if we assume a CPU core to cost 4 €/month.