[SPARK-4790][STREAMING] Fix ReceivedBlockTrackerSuite waits for old file...

...s to get deleted before continuing.

Since the deletes are happening asynchronously, the getFileStatus call might throw an exception in older HDFS
versions, if the delete happens between the time listFiles is called on the directory and getFileStatus is called
on the file in the getFileStatus method.

This PR addresses this by adding an option to delete the files synchronously and then waiting for the deletion to
complete before proceeding.
Can you extend this unit test to test for `waitForComplete = false` as well. Basically call `manager.cleanupOldLogs(manualClock.currentTime, waitForCompletion = false)` and see with `eventually` that more files gets deleted.