Have a proper java comment.
Model checkpoints and continuation for DRF.

Implemented:
- a method to fetch model producer key
- replay of OOB scores for DRF which is required for model continuation
Sharing code for tree scoring.
Test for DRF checkpointing.

The test verify that for checkpoint
it will reconstruct correct values in working "tree" columns
containing individual votes.
This is interesting only for OOB scoring.

The test cover multinomial, binomial, and regression models.
GBM in memory data structures reconstruction.
Minor fixes in DRF checkpointing code.
Checkpoint DRF test:
 - verify reconstructed results for checkpoint
 - verify created models if they produce same prediction
GBM checkpointing test
 - verify that reconstructed results from previous model are correct
 - verify prediction of the models
Fix a bug in reconstructing oob score to continue building drf model.

OOB scoring depends on RNG which needs to be
initialized correctly (based on checkpoint size - i.e., number of trees
generated in checkpoint).