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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.