cmd/mount: add max-downloads option to limit concurrent get for object storage

[jiefenghuang]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 69.33333% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.83%. Comparing base (2d372c7) to head (5664435).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/chunk/cached_store.go	66.66%	18 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6472      +/-   ##
==========================================
- Coverage   48.11%   44.83%   -3.28%     
==========================================
  Files         133      166      +33     
  Lines       42009    50861    +8852     
==========================================
+ Hits        20213    22806    +2593     
- Misses      19445    25339    +5894     
- Partials     2351     2716     +365     

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