[SPARK-2362] Fix for newFilesOnly logic in file DStream

The newFilesOnly logic should be inverted: the logic should be that if the flag newFilesOnly==true then only start reading files older than current time. As the code is now if newFilesOnly==true then it will start to read files that are older than 0L (that is: every file in the directory).

Author: Gabriele Nizzoli <mail@nizzoli.net>

Closes #1077 from gabrielenizzoli/master and squashes the following commits:

4f1d261 [Gabriele Nizzoli] Fix for newFilesOnly logic in file DStream

(cherry picked from commit e6f7bfcfbf6aff7a9f8cd8e0a2166d0bf62b0912)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>