[SPARK-12379][ML][MLLIB] Copy GBT implementation to spark.ml

Currently, GBTs in spark.ml wrap the implementation in spark.mllib. This is preventing several improvements to GBTs in spark.ml, so we need to move the implementation to ml and use spark.ml decision trees in the implementation. At first, we should make minimal changes to the implementation.
Performance testing should be done to ensure there were no regressions.

Performance testing results are [here](https://docs.google.com/document/d/1dYd2mnfGdUKkQ3vZe2BpzsTnI5IrpSLQ-NNKDZhUkgw/edit?usp=sharing)

Author: sethah <seth.hendrickson16@gmail.com>

Closes #10607 from sethah/SPARK-12379.