fix: Fixing the shutdown logic for streaming subscriber connection

[maitrimangal]

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Fixes #1544

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[hannahrogers-google]

I don't think we can do this. In runShutdown(), we call ackOperationsWaiter.waitComplete(). Which waits for all outstanding acks to be sent on the stub. If we close the stub before calling runShutdown, all remaining acks will all fail with the CancelledException.

Nevermind - I was thinking it was the stub we were shutting down. Shutting down the clientStream should be ok