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.
Interesting, I don't see this.  I also don't get any warnings prefaced with "WARNING" (in caps).  I wonder if it's because I'm running via sbt instead of using maven directly, or something like that.