Make retrieval even faster

[magik6k]

#7693 made retrieval quite fast, but we can do better

• Move piecereader closer to storage code (making it easier to add lower-level optimizations)
• Reuse one storage lock between piece seeks
• Reduce allocation pressure
• Don't do redundant sector finding / alloc checks
• (separate PR) mmaped reader for local sector access
• (separate PR) Cache reads in a temp file

[codecov]

Codecov Report

Merging #7746 (e8ef39e) into master (49c619d) will decrease coverage by 0.05%.
The diff coverage is 74.19%.

@@            Coverage Diff             @@
##           master    #7746      +/-   ##
==========================================
- Coverage   39.57%   39.52%   -0.06%     
==========================================
  Files         640      640              
  Lines       68410    68465      +55     
==========================================
- Hits        27074    27059      -15     
- Misses      36698    36751      +53     
- Partials     4638     4655      +17     

Impacted Files	Coverage Δ
extern/sector-storage/fr32/fr32.go	100.00% <ø> (+8.13%)	⬆️
extern/sector-storage/stores/http_handler.go	75.14% <ø> (-0.15%)	⬇️
extern/sector-storage/piece_reader.go	58.75% <50.00%> (ø)
extern/sector-storage/piece_provider.go	55.31% <66.66%> (-0.38%)	⬇️
markets/dagstore/miner_api.go	63.09% <70.00%> (ø)
extern/sector-storage/stores/remote.go	57.80% <72.22%> (-0.45%)	⬇️
markets/sectoraccessor/sectoraccessor.go	72.30% <75.00%> (ø)
extern/sector-storage/fr32/readers.go	81.25% <100.00%> (+1.25%)	⬆️
extern/sector-storage/mock/mock.go	66.27% <100.00%> (+0.90%)	⬆️
markets/dagstore/mocks/mock_lotus_accessor.go	80.48% <100.00%> (-0.47%)	⬇️
... and 24 more

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