[FLINK-2673] [type system] Add a comparator for Scala Option type

[chiwanpark]

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I've added OptionTypeComparator implementation, unit test cases based on ComparatorTestBase and a simple integration test case in JoinITCase. I followed Till's opinion about order between None and Some values.

[chiwanpark]

Failing Travis is not related to this PR.

[StephanEwen]

Thanks, @chiwanpark - looks good to me!

[chiwanpark]

Thanks for review @StephanEwen! Merging...