make an option to not reuse conns

[demmer]

Description

Make an option to not reuse conns.

This required some refactoring so that the interface between the proxy and the mysql server uses a struct that wraps the vtgateconn as well as the session, all so that we can call Close on the conn when the mysql server thread completes.

Related Issue(s)

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

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[github-actions]

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