[Fixed] Close subscriber on worker shutdown

[crimoniv]

🎩 What?

Try to achieve a graceful shutdown of the Worker by ensuring that SubscriberClient is properly closed, as we're still getting Cancelled messages even with the changes introduced in #295.

As an extra, this PR introduces some debug messages and test coverage (for this change and the previous one).

🤔 Why?

As per SubscriberClient documentation:

Must be used in a `with` block or have `__enter__()` called before use.

If we check the Python example provided in StreamingPull and high-level client library code samples, we can see that the subscriber is expected to the wrapped in a with block to automatically call close() when done:

# Wrap subscriber in a 'with' block to automatically call close() when done.
with subscriber:
    try:
        # When `timeout` is not set, result() will block indefinitely,
        # unless an exception is encountered first.
        streaming_pull_future.result(timeout=timeout)
    except TimeoutError:
        streaming_pull_future.cancel()  # Trigger the shutdown.
        streaming_pull_future.result()  # Block until the shutdown is complete.

🔗 Related issue

N/A