Process ad chain in segments during GC.

[gammazero]

If the amount of the ad chain to process is very long, then do not process all the ads at the same time. This may lead to keeping too many CIDs and context IDs in memory resulting in an out-of-memory condition. Instead process a segment of the chain at a time, starting from the oldest to the newest segment.

[codecov-commenter]

Codecov Report

Attention: 91 lines in your changes are missing coverage. Please review.

Comparison is base (4ee3e51) 47.06% compared to head (8d56ff9) 46.93%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
ipni-gc/reaper/reaper.go	10.52%	80 Missing and 5 partials ⚠️
ipni-gc/reaper/option.go	14.28%	6 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2460      +/-   ##
==========================================
- Coverage   47.06%   46.93%   -0.14%     
==========================================
  Files          85       85              
  Lines       10732    10737       +5     
==========================================
- Hits         5051     5039      -12     
- Misses       5080     5097      +17     
  Partials      601      601              

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