vtorc: use golang.org/x/sync/semaphore, add flag for db concurrency

[timvaillancourt]

Description

This PR migrates the concurrency control for VTOrc's backend to use golang.org/x/sync/semaphore and a flag is added to control the limit

Related Issue(s)

#17330

Checklist

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Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 67.44%. Comparing base (2118bc3) to head (0edc405).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/config/config.go	50.00%	4 Missing ⚠️
go/vt/vtorc/inst/instance_dao.go	75.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17837      +/-   ##
==========================================
- Coverage   67.45%   67.44%   -0.01%     
==========================================
  Files        1592     1592              
  Lines      258167   258227      +60     
==========================================
+ Hits       174143   174165      +22     
- Misses      84024    84062      +38     

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[GuptaManan100]

Rest is all good, but I don't think this field should be dynamic. Since we initialize the semaphore with value we read for this config once, even if the users change it later, the change won't be affected.

[GuptaManan100]

We didn't have a test before, but maybe its worth adding one to check that these concurrency controls are respected.