[SPARK-24267][SQL] explicitly keep DataSourceReader in DataSourceV2Relation#21319

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cloud-fan wants to merge 4 commits intoapache:masterfrom cloud-fan:refactor

Commits on May 14, 2018

do not create DataSourceReader many times
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Commits on May 15, 2018

address comments
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Commits on May 22, 2018

improve comments
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Commits on Jun 12, 2018

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