[SPARK-6515] update OpenHashSet impl

[mengxr]

Though I don't see any bug in the existing code, the update in this PR makes it read better. @rxin

[rxin]

LGTM.

[andrewor14]

LGTM2

[AmplabJenkins]

Test FAILed.
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[andrewor14]

retest this please

[SparkQA]

Test build #29131 has started for PR 5176 at commit 134494d.

• This patch merges cleanly.

[SparkQA]

Test build #29131 has finished for PR 5176 at commit 134494d.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

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[andrewor14]

Master