Throttler: refactor global configuration setting as throttler member

[shlomi-noach]

Description

Following recurring race condition failures, e.g. https://github.com/vitessio/vitess/actions/runs/7299836104/job/19894003119?pr=14851, this PR removes the global throttler configuration Instance, and makes it a member of the throttler struct. Each throttler has its own dedicated config settings now.

This eliminates race conditions in the unit tests, and generally plays nicer with unit testing.

Related Issue(s)

Closes #14852

Fixes: #14776

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.

[shlomi-noach]

Let's run Unit Test (Race) tests several times in this PR.

[mattlord]

Let's run Unit Test (Race) tests several times in this PR.

Sounds good! I usually try to do 15-20 in similar PRs (that seems to be pretty representative in my anecdotal experience). We can collectively hit the re-run button here: https://github.com/vitessio/vitess/actions/runs/7301655630/job/19898750909?pr=14853

[shlomi-noach]

Noting that there is no backport to this PR.

[shlomi-noach]

Related/unrelated, TestSuspendableTickerTick are also flaky. Will fix in a different PR though.