Fix MySQL mutex sometimes allows acquiring of the same locknames

[raphaelts3]

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Tests pass?	✔️
Fixed issues	https://github.com/yiisoft/yii2/issues/18877

Since @alexjeen hasn't answered about pull requesting the fix for the bug, I've taken the liberty of pull requesting his solution with a small code arrangement that I think improves overall readability. I've included both of us in the changelog because I think it's the more sensate thing to do, but let me know if I should change it.

[raphaelts3]

@samdark when you got some time I would your review here, thank you!