Fix mysql certificate serial

[Lagovas]

Last fix to make tests green, and back old mysql's serial to index database to fix issues related to OCSP checks

Checklist

• [x] Change is covered by automated tests
• [X] The coding guidelines are followed
• [X] Public API has proper documentation in the Acra documentation site or has PR on documentation repository with new changes
• [X] CHANGELOG.md is updated (in case of notable or breaking changes)
• [X] CHANGELOG_DEV.md is updated
• [X] Benchmark results are attached (if applicable)
• X are up-to-date (in case of API changes)