[SPARK-27070] Fix performance bug in DefaultPartitionCoalescer#23986

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fitermay wants to merge 4 commits intoapache:masterfrom

fitermay:SPARK-27070

Commits on Mar 6, 2019

SPARK-27070: Don't use sort to find minimum PartitionGroup
fitermay
committed

Commits on Mar 8, 2019

add benchmark and incorporate suggestion from review
fitermay
committed

Commits on Mar 10, 2019