[BEAM-3800] Set uids on Flink operators

[grzegorz8]

In most of the cases the transform's name is used as UID, since the transform's name is unique across the job. The uid is not set for PartitionTransformations (dataStream.keyBy(_)) and simple stateless flatMaps wrapping one object with another one.

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[aljoscha]

retest this please

[aljoscha]

Thanks for working on this! 😃