HEX-1711 Model persistence - export/import from HDFS/S3N/local FS

The commit includes:
  - model serialization method based on modified autobuffers
  - test for tree models
  - save/load requests loading from local file
  - fixes to correctly serialize GLM model.
  - serialization infrastructure for local FS and HDFS

The commit changes:
  - AutoBuffer is not final (since it is needed for model serialization)
  - job parameters are cloned inside model and passed through hygienic
    method which delete references to frames.