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.
Oh oh, btw, we should never catch Throwable in Scala (learnt about this recently). Scala subsystem uses a number of stuff throwable for control flow and catching throwables can give rise to very weird scenarios. Only exceptions should be caught, and reported, and all useful catchable things are mostly exceptions from what i have ever seen.

For examples, OOMs are throwables but not exceptions and should never be caught.