[KYLIN-4827] Fix SparkMergingDictionary parallelize not work.

[Ted-Jiang]

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[coveralls]

Pull Request Test Coverage Report for Build 6581

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• 12 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.006%) to 28.043%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
engine-spark/src/main/java/org/apache/kylin/engine/spark/SparkMergingDictionary.java	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
core-dictionary/src/main/java/org/apache/kylin/dict/lookup/cache/RocksDBLookupTable.java	1	81.08%
tool/src/main/java/org/apache/kylin/tool/query/ProbabilityGenerator.java	1	81.58%
core-cube/src/main/java/org/apache/kylin/cube/cuboid/TreeCuboidScheduler.java	2	68.46%
core-job/src/main/java/org/apache/kylin/job/impl/threadpool/DefaultScheduler.java	2	79.07%
stream-core/src/main/java/org/apache/kylin/stream/core/storage/CheckPointStore.java	2	73.74%
core-cube/src/main/java/org/apache/kylin/cube/inmemcubing/MemDiskStore.java	4	78.12%

Totals
Change from base Build 6578:	-0.006%
Covered Lines:	26253
Relevant Lines:	93618

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💛 - Coveralls

[shaofengshi]

How much performance improvement from this change? I'd like to know it. Thanks!

[Ted-Jiang]

@shaofengshi In sample cube, merge two month data, did not seen any promote. But in production env it can fix GC job failed issue.

[shaofengshi]

thanks Ted, it looks good to me!