[SPARK-4518][SPARK-4519][Streaming] Refactored file stream to prevent files from being processed multiple times

Because of a corner case, a file already selected for batch t can get considered again for batch t+2. This refactoring fixes it by remembering all the files selected in the last 1 minute, so that this corner case does not arise. Also uses spark context's hadoop configuration to access the file system API for listing directories.
Also does it make sense to move this to an external class, not a inner class?