[FLINK-18082][jdbc] UnsignedTypeConversionITCase stalls in ch.vorburer.mariadb4j.DB.stop

[leonardBang]

What is the purpose of the change

• This pull request try to fix unstable UnsignedTypeConversionITCase which use mariadb4j

Brief change log

• Bump test dependency mariadb-java-client from version 2.3.0 to 2.5.3 which fix a few notable bugs mariadb-java-client release note, and the lower version may cause the test stability issue.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): ( no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Automated Checks

Last check on commit afb8eed (Wed Jun 03 13:25:25 UTC 2020)

Warnings:

• 1 pom.xml files were touched: Check for build and licensing issues.
• No documentation files were touched! Remember to keep the Flink docs up to date!

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• ❓ 5. Overall code [quality] is good.

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• afb8eed Azure: SUCCESS

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[wuchong]

LGTM and merged.