[SPARK-22087][SPARK-14650][WIP][BUILD][REPL][CORE] Compile Spark REPL for Scala 2.12 + other 2.12 fixes

## What changes were proposed in this pull request?

Enable Scala 2.12 REPL. Fix most remaining issues with 2.12 compilation and warnings, including:

- 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

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #19307 from srowen/Scala212.