[FLINK-18403][checkpointing] Ensure that unaligned checkpointing is only activated for EXACTLY_ONCE.

[AHeise]

What is the purpose of the change

Currently, unaligned checkpoint config option can be used together with AT_LEAST_ONCE (or disabled checkpointing), which may lead to unexpected behavior.
This PR ensures that unaligned checkpointing is only activated for EXACTLY_ONCE.

Brief change log

• Deactivates unaligned checkpointing if checkpointing mode is not EXACTLY_ONCE and gives warning.

Verifying this change

Added two unit tests.

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

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Automated Checks

Last check on commit 633d0f7 (Mon Jun 22 09:56:43 UTC 2020)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

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Review Progress

• ❓ 1. The [description] looks good.
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• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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[pnowojski]

LGTM - Could you ping me once azure is green?

[flinkbot]

CI report:

• 633d0f7 Azure: SUCCESS

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