irmin-pack: implement reading from lower

[metanivek]

This PR:

• adds a read_exn function to Lower
• limits "open" volumes to one at a time for a simple management of open file descriptors
• adds volume_identifier to direct keys to allow tracking the context of a read so that subsequent reads can skip looking in the upper
• handles dangling parent commits by not propagating the volume identifier to them, thereby forcing a fresh lookup

[codecov-commenter]

Codecov Report

Merging #2198 (656c1bf) into main (43b8a6c) will increase coverage by 0.04%. The diff coverage is 72.64%.

:exclamation: Current head 656c1bf differs from pull request most recent head b4571a8. Consider uploading reports for the commit b4571a8 to get more accurate results

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@@            Coverage Diff             @@
##             main    #2198      +/-   ##
==========================================
+ Coverage   68.03%   68.07%   +0.04%     
==========================================
  Files         135      135              
  Lines       16301    16366      +65     
==========================================
+ Hits        11090    11141      +51     
- Misses       5211     5225      +14     

Impacted Files	Coverage Δ
src/irmin-pack/unix/errors.ml	28.88% <ø> (ø)
src/irmin-pack/unix/io_errors.ml	58.33% <ø> (ø)
src/irmin-pack/unix/store.ml	64.70% <ø> (+0.11%)	:arrow_up:
src/irmin-pack/unix/lower.ml	69.69% <60.00%> (-11.01%)	:arrow_down:
src/irmin-pack/unix/sparse_file.ml	78.70% <76.92%> (+8.61%)	:arrow_up:
src/irmin-pack/unix/dispatcher.ml	60.00% <82.35%> (+3.93%)	:arrow_up:
src/irmin-pack/unix/pack_key.ml	66.03% <83.33%> (+0.65%)	:arrow_up:
src/irmin-pack/unix/pack_store.ml	79.02% <93.33%> (+0.41%)	:arrow_up:
src/irmin-pack/unix/checks.ml	23.28% <100.00%> (ø)
src/irmin-pack/unix/file_manager.ml	84.43% <100.00%> (ø)
... and 9 more

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[metanivek]

Just had a fun rebase but can merge when CI is green