[SPARK-25299] Move shuffle writers back to being given specific partition ids #540
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We originally made the shuffle map output writer API behave like an iterator in fetching the "next" partition writer. However, the shuffle writer implementations tend to skip opening empty partitions. If we used an iterator-like API though we would be tied down to opening a partition writer for every single partition, even if some of them are empty. Here, we go back to using specific partition identifiers to give us more freedom to avoid needing to create writers for empty partitions.