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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[codecov]

Codecov Report

Attention: Patch coverage is 65.21739% with 8 lines in your changes missing coverage. Please review.

Project coverage is 67.47%. Comparing base (2118bc3) to head (ea07036).
Report is 23 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	73.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17837      +/-   ##
==========================================
+ Coverage   67.45%   67.47%   +0.02%     
==========================================
  Files        1592     1593       +1     
  Lines      258167   258903     +736     
==========================================
+ Hits       174143   174702     +559     
- Misses      84024    84201     +177     

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[GuptaManan100]

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

[timvaillancourt]

@GuptaManan100 sounds good 👍

I've been optimizing code as I come across it lately and only now I see ExecDBWriteFunc is used only in a handful of write operations and instanceReadChan/instanceReadSem is never used

So some more cleanup is needed. Do we want all reads/writes to respect a limit? And considering a majority of reads/writes in VTOrc aren't respecting a limit, do we need one? 🤔

[GuptaManan100]

This is fine too, but if we want to make all the read and write calls acqurie this lock too, then that would technically be more correct.

[timvaillancourt]

@GuptaManan100 I'm torn, I suspect this semaphore made more sense when the backend was remote and not in-memory

For sqlite3 there is a single writer so I think it already will constrain concurrency, so potentially only a read limit will be useful 🤔

I merged this PR too soon when I saw it had 2 x reviews, but I can follow up with whatever we decide

[deepthi]

Nice improvement!