Enhance sinks to support idleness.

[EronWright]

What is the purpose of the change

Propagates the idleness signal to the SinkFunction and SinkWriter for connector-specific purposes such as watermark propagation across pipelines.

Brief change log

• Add processStreamStatus() method to StreamOperator interface with default implementation
• Update abstract stream operator to invoke processStreamStatus when the combined input changes.
• Add markIdle() method to SinkFunction interface
• Add markIdle() method to SinkWriter interface
• Add support to sink operators for invoking markIdle() on sink functions.

Verifying this change

(TODO) This change adds tests for idleness callbacks, and verifies exception handling behavior.

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): yes
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? JavaDocs