Throttler code: some refactoring and cleanup

[shlomi-noach]

Description

This is a quick followup to #15988. This PR includes no logic changes. It's all about changing some names, and moving some files. Specifically, removing "MySQL" from a lot of function and variable names, where it's just irrelevant. Removing the mysql package as it has nothing specific to MySQL and converging it into the throttle/base package.

Other than the refactor of names, the tests are unchanges and again no logic changes were made.

Related Issue(s)

#15624

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 81.10236% with 24 lines in your changes missing coverage. Please review.

Project coverage is 68.68%. Comparing base (75aee8a) to head (a8eeea7).
Report is 3 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/tabletserver/throttle/throttler.go	82.10%	17 Missing ⚠️
...abletserver/throttle/base/throttle_metric_cache.go	65.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16368      +/-   ##
==========================================
- Coverage   68.68%   68.68%   -0.01%     
==========================================
  Files        1548     1548              
  Lines      199081   199063      -18     
==========================================
- Hits       136747   136733      -14     
+ Misses      62334    62330       -4     

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[rohit-nayak-ps]

Should be 2024?

[shlomi-noach]

fixed

[rohit-nayak-ps]

No need for this attribution anymore?

[shlomi-noach]

Not in this specific file. The code in this file is not derived from freno, and the note was needlessly copy+pasted from another file.