SPARK-1582 Invoke Thread.interrupt() when cancelling jobs

Sometimes executor threads are blocked waiting for IO or monitors, and the current implementation of job cancellation may never recover these threads. By simply invoking Thread.interrupt() during cancellation, we can often safely unblock the threads and use them for subsequent work.

Note that this feature must remain optional for now because of a bug in HDFS where Thread.interrupt() may cause nodes to be marked as permanently dead (as the InterruptedException is reinterpreted as an IOException during communication with some node).