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.
Looks like we'd still need a lock, since we have to do a set of ops atomically (check if the queue has anything, then pop else check that currentSize < maxInstances and create a new instance - all this needs to be done atomically, else it is possible that sizes can grow beyond maxInstances).  Using a blocking queue that way would end up being more expensive.