Improve memory efficiency in FnApiDoFnRunner

[stankiewicz]

Reduces memory footprint of storing map of coders, pcollections and windowStrategies within FnApiDoFnRunner by reusing the immutable component created once per stage vs copied recursively for earch transform.

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R: @m-trieu for label java.

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[damccorm]

@m-trieu would you mind taking a look at this one?

[kennknowles]

Makes sense to me.

[kennknowles]

It is a small change that should impact all pipelines, so let's watch the postcommit test status. It seems pretty safe though so I think it is an adequate level of care to merge and watch how things go.