Fix PRS from being blocked because of misbehaving clients

[GuptaManan100]

Description

This PR fixes the problem described in #14760. If a client stops reading messages from a stream query, we can't really prevent vtgates from continuing to be blocked.
What we can do is unblock vttablets from getting stuck at demote primary calls if shutdown grace period has been specified.

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 period 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.

This PR is a rework of #14761 with additional tests and refactoring to make the code more readable.

The default for the flag shutdown_grace_period has also been changed. The flag was introduced first in #2301 and has defaulted to 0 always. Changing the default allows Vitess clusters that start with default values to not block PRS on client errors.

Related Issue(s)

• Fixes Bug Report: Demote Primary stuck #14760

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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Deployment Notes

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.42%. Comparing base (696fe0e) to head (9c4a881).
Report is 63 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15339      +/-   ##
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- Coverage   67.41%   65.42%   -2.00%     
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  Files        1560     1562       +2     
  Lines      192752   193660     +908     
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- Hits       129952   126701    -3251     
- Misses      62800    66959    +4159     

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[harshit-gangal]

overall looks good. Just unsure about setting the shutdown grace period value to a low default.

[deepthi]

Release notes need editing. Rest LGTM