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

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

streaming/src/main/scala/org/apache/spark/streaming/dstream/FileInputDStream.scala

@@ -45,7 +45,7 @@ class FileInputDStream[K: ClassTag, V: ClassTag, F <: NewInputFormat[K,V] : Clas
   // Files with mod time earlier than this is ignored. This is updated every interval
   // such that in the current interval, files older than any file found in the
   // previous interval will be ignored. Obviously this time keeps moving forward.
-  private var ignoreTime = if (newFilesOnly) 0L else System.currentTimeMillis()
+  private var ignoreTime = if (newFilesOnly) System.currentTimeMillis() else 0L
 
   // Latest file mod time seen till any point of time
   @transient private var path_ : Path = null