Refactor vtgate.Executor

[systay]

Description

It's been a while since we spent some quality time cleaning up around our vtgate Executor.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• 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]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 68.96552% with 9 lines in your changes missing coverage. Please review.

Project coverage is 67.04%. Comparing base (e881b9f) to head (cb90cc9).
Report is 83 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/vtgate.go	0.00%	7 Missing ⚠️
go/vt/vtgate/vcursor_impl.go	75.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16993   +/-   ##
=======================================
  Coverage   67.04%   67.04%           
=======================================
  Files        1571     1571           
  Lines      251677   251688   +11     
=======================================
+ Hits       168729   168740   +11     
  Misses      82948    82948           

☔ View full report in Codecov by Sentry.
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[github-actions]

This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

• Push additional commits to the associated branch.
• Remove the stale label.
• Add a comment indicating why it is not stale.

If no action is taken within 7 days, this PR will be closed.