s3: upload package files concurrently

[arges]

Upload package files in bounded concurrent batches while keeping metadata generation and conflicting paths ordered. Default to four uploads and allow per-endpoint configuration.

Fixes #1615

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Also, to speed up things, if you could kindly "Allow edits and access to secrets by maintainers" in the
PR settings, as this allows us to rebase the PR on master, fix conflicts, run coverage and help with
implementing code and tests.

Description of the Change

This allows for s3 uploads to be done in parallel making uploads work much faster.

Checklist

• allow Maintainers to edit PR (rebase, run coverage, help with tests, ...)
• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[codecov]

Codecov Report

❌ Patch coverage is 69.13580% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.04%. Comparing base (f59b0d2) to head (2d44e00).

Files with missing lines	Patch %	Lines
deb/publish.go	65.67%	12 Missing and 11 partials ⚠️
context/context.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1616      +/-   ##
==========================================
- Coverage   77.37%   77.04%   -0.34%     
==========================================
  Files         165      165              
  Lines       15747    15805      +58     
==========================================
- Hits        12185    12177       -8     
- Misses       2356     2417      +61     
- Partials     1206     1211       +5     

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