Improved logic of finding new files in FileInputDStream

Earlier, if HDFS has a hiccup and reports a existence of a new file (mod time T sec) at time T + 1 sec, then fileStream could have missed that file. With this change, it should be able to find files that are delayed by up to <batch size> seconds. That is, even if file is reported at T + <batch time> sec, file stream should be able to catch it. 

The new logic, at a high level, is as follows. It keeps track of the new files it found in the previous interval and mod time of the oldest of those files (lets call it X). Then in the current interval, it will ignore those files that were seen in the previous interval and those which have mod time older than X. So if a new file gets reported by HDFS that in the current interval, but has mod time in the previous interval, it will be considered. However, if the mod time earlier than the previous interval (that is, earlier than X), they will be ignored. This is the current limitation, and future version would improve this behavior further.