Implement fully asynchronous Sinks and graceful pod shutdown (Java runtime)

[eolivelli]

Summary:

• rework the internal Sink API to be fully async
• Kafka Topic Sink
• Kafka Connect Sinks
• Cassandra Sink
• Pinecone Sink (not really async)
• now that everything is async we have to wait for all the messages produced by the Source to be processed before shutting down the process (or the test)

[eolivelli]

This is still work-in-progress.

Now that everything is fully async some tests are flaky because we don't wait for all the messages to be passed to the sink and completely processed

[eolivelli]

I have tested the patch manually under some load.
It is safe to commit it