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.
This does not eliminate the warning for me:
```
[warn] .../spark/sql/core/src/test/scala/org/apache/spark/sql/parquet/ParquetQuerySuite.scala:461: abstract type T is unchecked since it is eliminated by erasure
```