feat: Attempted implemented graceful shutdown

[jjm340]

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Fixes #<issue_number_goes_here> 🦕

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[feywind]

@jjm340 Thanks for opening a PR. I think I can see some ways we can simplify this, but my main concern is that it might be considered a breaking change. I'd say "let me talk to the team on Thursday" except it's going to be US Thanksgiving :) so many of them will be out. Anyhow, I'll get back to you.

[jjm340]

@jjm340 Thanks for opening a PR. I think I can see some ways we can simplify this, but my main concern is that it might be considered a breaking change. I'd say "let me talk to the team on Thursday" except it's going to be US Thanksgiving :) so many of them will be out. Anyhow, I'll get back to you.

@feywind Thanks!! I had the same thought about this being a breaking change and a colleague and I discussed putting this behind an option flag, but wanted feedback from Google on that. I believe one side effect is that if this approach is used and a message is neither ACKed or NACKed, then the promise would never resolve and keep the consumer open indefinitely.