[FLINK-22184][client] Shutdown client outside of netty thread

[zentol]

If the future returned by clusterClient.submitJob() fails, then this failure is executed by a netty thread. This then causes .whenComplete((ignored1, ignored2) -> clusterClient.close()) to also be run in the netty thread, which causes the shutdown to fail because netty can't be shutdown since a thread is still active.

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[tillrohrmann]

Thanks for creating this PR @zentol. LGTM. +1 for merging.