XD-1278 Renaming avro sink to hdfs-dataset

- Adding support for parquet in addition to avro
  - use new option --format=parquet
  - for this option to work the payload currently MUST be a JavaBean with properties for all fields to be stored

- Adding support for not allowing nulls
  - this avoids generating an Avro UNION for every field in the schema
  - use new option --allowNullValues=false