Decide whether spark.ml Decision Tree and Random Forest can replace spark.mllib implementation

This JIRA is for discussing replacing the spark.mllib DecisionTree and RandomForest implementations with the implementation in spark.ml. The new implementation is simply a copy, with slight modifications (removing "bins").

Pros:

• Support only 1 implementation.
• Efficiency gains in spark.ml will benefit both APIs.

Cons:

• As spark.ml tree functionality increases, we will need to maintain conversion code for converting spark.ml trees to spark.mllib trees.

Must:

• Ensure we do not have significant regressions in the new implementation.