Implement concurrent downloads

[jwodder]

Closes #3.
Closes #8.
Closes #15.

[codecov]

Codecov Report

Attention: Patch coverage is 2.19298% with 223 lines in your changes missing coverage. Please review.

Project coverage is 29.87%. Comparing base (3b2d603) to head (bb266b3).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
src/syncer.rs	0.00%	129 Missing ⚠️
src/asyncutil/lsg.rs	0.00%	38 Missing ⚠️
src/s3/location.rs	21.73%	18 Missing ⚠️
src/inventory/list.rs	0.00%	15 Missing ⚠️
src/s3/mod.rs	0.00%	12 Missing ⚠️
src/inventory/item.rs	0.00%	7 Missing ⚠️
src/main.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #23       +/-   ##
===========================================
- Coverage   40.36%   29.87%   -10.49%     
===========================================
  Files          10       13        +3     
  Lines         545      743      +198     
===========================================
+ Hits          220      222        +2     
- Misses        325      521      +196     

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