reduce memory consumption of files cache, fixes #5756

[ThomasWaldmann]

• refactor packing/unpacking of fc entries into separate functions
• instead of a chunks list entry being a tuple of 256bit id [bytes] and 32bit size [int],
  only store a stable 32bit index into kv array of ChunkIndex (where we also have id and
  size [and refcount]).
• only done in memory, the on-disk format has (id, size) tuples.

memory consumption (N = entry.chunks list element count, X = overhead for rest of entry):

• previously:
  • packed = packb(dict(..., chunks=[(id1, size1), (id2, size2), ...]))
  • packed size ~= X + N * (1 + (34 + 5)) Bytes
• now:
  • packed = packb(dict(..., chunks=[ix1, ix2, ...]))
  • packed size ~= X + N * 5 Bytes

[codecov]

Codecov Report

Attention: Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.75%. Comparing base (1ec66ed) to head (26bb782).
Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
src/borg/cache.py	97.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8511      +/-   ##
==========================================
+ Coverage   81.43%   81.75%   +0.32%     
==========================================
  Files          73       73              
  Lines       13108    13139      +31     
  Branches     1924     1926       +2     
==========================================
+ Hits        10674    10742      +68     
+ Misses       1769     1740      -29     
+ Partials      665      657       -8     

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