[BEAM-5797] Ensure ExecutableStageDoFnOperator dispose is executed once

[tweise]

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---			---	---	---

[tweise]

CC: @mwylde

[mxm]

Kind of unexpected that dispose can be called twice. Looked into the Flink code (StreamTask), and indeed it calls once without catching exceptions and, if exceptions occurred, another time where it catches exceptions and logs them.