Parallelize FileBlob deletion in cleanup

[caseyduquettesc]

Fixes #67545

cleanup_unused_files can take over an hour to run regardless of the concurrency setting. This creates new processes for deleting unused files in parallel. In my own install, I've observed this process now taking 35 minutes instead of 60 with concurrency set to 4, making this more scalable for us.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.32%. Comparing base (6bdbae6) to head (1f64dca).
Report is 1060 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #68556      +/-   ##
==========================================
+ Coverage   71.87%   79.32%   +7.45%     
==========================================
  Files        6402     6395       -7     
  Lines      283333   283455     +122     
  Branches    48812    48824      +12     
==========================================
+ Hits       203653   224860   +21207     
+ Misses      79319    58239   -21080     
+ Partials      361      356       -5     

see 1144 files with indirect coverage changes

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[caseyduquettesc]

bump