[BEAM-4150] Augment Python SDK worker to handle unwindowed coders.

[robertwb]

DESCRIPTION HERE

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[robertwb]

R: @axelmagn, @aljoscha . This should help with some of the errors when trying to run on Flink.

[herohde]

@robertwb Is there a JIRA for this work? (I'm asking assuming similar work is needed for the Go SDK)

[robertwb]

I couldn't find a JIRA, so I created one.

@herohde yes, Go will need to do the same (if it doesn't already).

@axelmagn, could you please take a look?

[herohde]

Thanks @robertwb -- it does this as part of the Go windowing PR.

[robertwb]

Jenkins: retest this please.