SPARK-1729. Make Flume pull data from source, rather than the current pu...

...sh model

Currently Spark uses Flume's internal Avro Protocol to ingest data from Flume. If the executor running the
receiver fails, it currently has to be restarted on the same node to be able to receive data.

This commit adds a new Sink which can be deployed to a Flume agent. This sink can be polled by a new
DStream that is also included in this commit. This model ensures that data can be pulled into Spark from
Flume even if the receiver is restarted on a new node. This also allows the receiver to receive data on
multiple threads for better performance.
I had to change this a bit, since Avro will serialize only fields represented by the message and not new fields in the subclass. Avro also does not support inheritance, so I added an additional field in the batch to show if it is an error or not. If the value is empty, then it means that it is a valid batch. If it is not, that value is used as the error message in the log on the Spark side