Add new mysql connection drain

[frouioui]

Description

This PR implements the #15971 RFC by adding a new mysql connection drain, which comes as an alternative to the already existing activity drain timeout.

Related Issue(s)

• RFC: Drain MySQL connection in VTGate upon termination of VTGate #15971

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

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 28 lines in your changes missing coverage. Please review.

Project coverage is 68.71%. Comparing base (bb76046) to head (f1b0d53).
Report is 6 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/plugin_mysql_server.go	0.00%	27 Missing ⚠️
go/vt/servenv/run.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16298      +/-   ##
==========================================
- Coverage   68.71%   68.71%   -0.01%     
==========================================
  Files        1544     1547       +3     
  Lines      198011   198280     +269     
==========================================
+ Hits       136064   136244     +180     
- Misses      61947    62036      +89     

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[harshit-gangal]

listener.Shutdown() calls listener. Close(). Any reason for doing this?

[frouioui]

listener.Shutdown() also set the .shutdown field to true and then close. We only want to close the listener here to stop accepting new connection but let the idle/current connection active. Shutting down will fail any idle connection on the next query.

[harshit-gangal]

There should be more tests.

1. Trying to use connection beyond on_term timeout.
2. Run a busy query on a connection that returns the result after on_term timeout.

[frouioui]

There should be more tests.

1. Trying to use connection beyond on_term timeout.
2. Run a busy query on a connection that returns the result after on_term timeout.

Done via 1f7dfd3 cc @harshit-gangal

[frouioui]

Now that I merged this, I realized we could use some release notes for this. I will add them in a subsequent PR.