keyedTimeBatchWindowTest got a CME

The keyedTimeBatchWindowTest got a CME (vintage 2016-09-28 code) in a travis-ci build.

Note an earlier PR-172 related to this test: https://github.com/apache/incubator-edgent/pull/172

Below, line 425 refers to:
assertTrue("Values:" + batch.toString(), withinTolerance(1000.0, l.doubleValue(), tolerance));

Perhaps a wait or delay is needed following the sf.cancel(true)?

[junit] Testcase: keyedTimeBatchWindowTest(org.apache.edgent.test.window.WindowTest): Caused an ERROR
[junit] null
[junit] java.util.ConcurrentModificationException
[junit] at java.util.LinkedList$ListItr.checkForComodification(LinkedList.java:966)
[junit] at java.util.LinkedList$ListItr.next(LinkedList.java:888)
[junit] at java.util.AbstractCollection.toString(AbstractCollection.java:461)
[junit] at org.apache.edgent.test.window.WindowTest.keyedTimeBatchWindowTest(WindowTest.java:425)
[junit]