Clear remaining compile errors for 2.12; resolve most warnings

In the course of finishing SPARK-14650 I found there are still some compile problems and tens of avoidable warnings in 2.12 that can be fixed without, it seems, impacting the 2.11 build. There's enough work that I made a separate JIRA for it here.

It generally includes:

• Selecting Kafka 0.10.1+ for Scala 2.12 and patching over a minor API difference
• Fixing lots of "eta expansion of zero arg method deprecated" warnings
• Resolving the SparkContext.sequenceFile implicits compile problem
• Fixing an odd but valid jetty-server missing dependency in hive-thriftserver