[SPARK-25837] [Core] Fix potential slowdown in AppStatusListener when cleaning up stages#22883

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patrickbrownsync wants to merge 3 commits intoapache:masterfrom

Blyncs:cleanup-stages-fix

Commits on Oct 29, 2018

Update cleanupStages in AppStatusListener to delete tasks for all stages in a single pass
patrickbrownsync
committed
remove uneeded type
patrickbrownsync
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Commits on Oct 31, 2018

fix style issue
patrickbrownsync
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