[SPARK-1749] Job cancellation when SchedulerBackend does not implement killTask

It turns out that having the DAGScheduler tell the taskScheduler to cancelTasks when the backend does not implement killTask (e.g. Mesos) is not such a good idea.
Do we have the meaning of all the listener events fully documented someplace?  Or perhaps that needs to be done in a separate PR and then DAGScheduler updated to match the documented expectation?