[SPARK-7356] [STREAMING] Fix flakey tests in FlumePollingStreamSuite using...

[SPARK-7356] [STREAMING] Fix flakey tests in FlumePollingStreamSuite using SparkSink's batch CountDownLatch.

This is meant to make the FlumePollingStreamSuite deterministic. Now we basically count the number of batches that have been completed - and then verify the results rather than sleeping for random periods of time.

Author: Hari Shreedharan <hshreedharan@apache.org>

Closes #5918 from harishreedharan/flume-test-fix and squashes the following commits:

93f24f3 [Hari Shreedharan] Add an eventually block to ensure that all received data is processed. Refactor the dstream creation and remove redundant code.
1108804 [Hari Shreedharan] [SPARK-7356][STREAMING] Fix flakey tests in FlumePollingStreamSuite using SparkSink's batch CountDownLatch.