SPARK-4297 [BUILD] Build warning fixes omnibus

There are a number of warnings generated in a normal, successful build right now. They're mostly Java unchecked cast warnings, which can be suppressed. But there's a grab bag of other Scala language warnings and so on that can all be easily fixed. The forthcoming PR fixes about 90% of the build warnings I see now.
That warning does not show up for me.  Do you need some higher-than-default warning setting turned on for it?  (The fix does seem fine; I'm just wondering.)