Enhance memory management

[benoit74]

Fix #38

This PR introduces two changes to reduce memory consumption:

• allow to customize number of libzim workers
• apply backpressure when adding items to the ZIM

[codecov]

Codecov Report

❌ Patch coverage is 17.24138% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 30.30%. Comparing base (327d44d) to head (08ae6c2).
⚠️ Report is 34 commits behind head on main.

Files with missing lines	Patch %	Lines
scraper/src/maps2zim/processor.py	14.81%	23 Missing ⚠️
scraper/src/maps2zim/entrypoint.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
- Coverage   30.32%   30.30%   -0.02%     
==========================================
  Files          11       11              
  Lines        1052     1069      +17     
  Branches      138      141       +3     
==========================================
+ Hits          319      324       +5     
- Misses        722      734      +12     
  Partials       11       11              

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[benoit74]

This is dead-locking, let's focus on openzim/python-scraperlib#282 and #40 for the time being

[benoit74]

Management of inflight items proved to not be an issue at all, issue was simply redirects handling in libzim