[SPARK-3160] [mllib]  DecisionTree: eliminate pre-allocated nodes, parentImpurities arrays

This PR includes some code simplifications and re-organization which will be helpful for implementing random forests.  The main changes are that the nodes and parentImpurities arrays are no longer pre-allocated in the main train() method.

Relation to RFs:
* Since RFs will be deeper and will therefore be more likely sparse (not full trees), it could be a cost savings to avoid pre-allocating a full tree.
* The associated re-organization also reduces bookkeeping, which will make RFs easier to implement.
* The return code doneTraining may be generalized to include cases such as nodes ready for local training.

Details:

No longer pre-allocate parentImpurities array in main train() method.
* parentImpurities values are now stored in individual nodes (in Node.stats.impurity).
* These were not really needed.  They were used in calculateGainForSplit(), but they can be calculated anyways using parentNodeAgg.

No longer using Node.build since tree structure is constructed on-the-fly.
* Did not eliminate since it is public (Developer) API.  Marked as deprecated.

Eliminated pre-allocated nodes array in main train() method.
* Nodes are constructed and added to the tree structure as needed during training.
* Moved tree construction from main train() method into findBestSplitsPerGroup() since there is no need to keep the (split, gain) array for an entire level of nodes.  Only one element of that array is needed at a time, so we do not the array.

findBestSplits() now returns 2 items:
* rootNode (newly created root node on first iteration, same root node on later iterations)
* doneTraining (indicating if all nodes at that level were leafs)

Updated DecisionTreeSuite.  Notes:
* Improved test "Second level node building with vs. without groups"
** generateOrderedLabeledPoints() modified so that it really does require 2 levels of internal nodes.
* Related update: Added Node.deepCopy (private[tree]), used for test suite

CC: @mengxr