[SPARK-3042] [mllib] DecisionTree Filter top-down instead of bottom-up

DecisionTree needs to match each example to a node at each iteration.  It currently does this with a set of filters very inefficiently: For each example, it examines each node at the current level and traces up to the root to see if that example should be handled by that node.

Fix: Filter top-down using the partly built tree itself.

Major changes:
* Eliminated Filter class, findBinsForLevel() method.
* Set up node parent links in main loop over levels in train().
* Added predictNodeIndex() for filtering top-down.
* Added DTMetadata class

Other changes:
* Pre-compute set of unorderedFeatures.

Notes for following expected PR based on [https://issues.apache.org/jira/browse/SPARK-3043]:
* The unorderedFeatures set will next be stored in a metadata structure to simplify function calls (to store other items such as the data in strategy).

I've done initial tests indicating that this speeds things up, but am only now running large-scale ones.

CC: @mengxr @manishamde @chouqin  Any comments are welcome---thanks!
It would be helpful if we add doc for `numClasses` mentioning its value for regression problems.