[FLINK-26708] TimestampsAndWatermarksOperator should not propagate WatermarkStatus

[dawidwys]

What is the purpose of the change

The lifecycle/scope of WatermarkStatus is tightly coupled with watermarks. Upstream watermarks are cut off in the TimestampsAndWatermarksOperator and therefore watermark statuses should be cut off as well.

Verifying this change

Added tests in:

• TimestampsAndWatermarksOperatorTest

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, 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)

[flinkbot]

CI report:

• 01f427f Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[dawidwys]

@flinkbot run azure

[smattheis]

Perfect. Thanks.