[SPARK-27340][SS] Alias on TimeWindow expression may cause watermark metadata lost#24457

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Commits on Apr 25, 2019

when use Column.name(alias: String), we create Alias directly with None explicitMetadata, so the metadata of Alias object will directly got from its child
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modify comments
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Commits on Apr 26, 2019

add ut
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Commits on Apr 28, 2019

modify code style
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