Scala Compiler Warnings

[jbaiera]

We should probably not tolerate compiler warnings in Scala any longer, especially since there's only a handful that I can see in the project (currently 13 in each spark project 1.6 & 2.0). This is a meta ticket for fixing compiler warnings in Scala, and after this ticket is completed, the compiler should be configured with '-Xfatal-warnings' to ensure no more compiler warnings are committed going forward (as they will fail the build).

• Fix discarded return value Scala compiler warnings #970 Fix discarded return value Scala compiler warnings
• Fix unchecked type Scala compiler warnings. #971 Fix unchecked type Scala compiler warnings.
• Fix dead code Scala compiler warnings #972 Fix dead code Scala compiler warnings
• Fix implicit conversion method Scala compiler warnings #973 Fix implicit conversion method Scala compiler warnings
• Enable Fatal Warnings on Scala Compiler #974 Enable Fatal Warnings on Scala Compiler

[takezoe]

I can work for them.

[jbaiera]

Closing this issue out as all sub-items have been closed out or merged.