Spark 2 compatibility

[tashoyan]

As Spark 2 is not a breaking news anymore, it's time to make reference apps working with it.
I can see at least the following things to do:

• Use SparkSession instead of SQLContext
• Use Dataset API
• Update JavaDStream.foreachRDD() invocations to new contract (returns nothing)
• Update JavaPairDStream.updateStateByKey() to new contract (uses Spark implementation of Optional)

[tashoyan]

I can handle Java part.
The question: do we still need to maintain Java 6 compatible code? Can we exclude support for this outdated Java version?