Rework unserveCommon so that we don't wait for requests always

[GuptaManan100]

Description

One of the fixes to the problem described in #14760 is implemented in this PR. We can't really prevent vtgates from continuing to be blocked. What we can do is unblock vttablets from getting stuck and therefore not block demote primary calls.

The order of operations in vttablets are as follows when we call DemotePrimary -

1. We change the wanted state of vttablet, which prevents new queries from being accepted by the vttablet.
2. Next we kill all the oltp queries.
3. We wait for requests to be empty and kill all the olap queries after a grace perios has passed (configurable and not necessary)

In this PR, we are reworking this flow and making it so that we don't wait for the requests to be empty if we have killed all the queries against MySQL. This will unblock DemotePrimary and it is safe to do, because MySQL is guaranteed to be not running any queries.

Related Issue(s)

• Fixes Bug Report: Demote Primary stuck #14760

Checklist

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

[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.

[github-actions]

This PR was closed because it has been stale for 7 days with no activity.