[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.
I'll just take care of it... thanks mark!

Task killing is not supported in the fine-grained mode on mesos because, in that mode, we use Mesos's built in support for all of the control plane messages relating to tasks. So we'll have to figure out how to support killing tasks in that model. There are two questions, one is who actually sends the "kill" message to the executor and the other is how we tell Mesos that the cores are freed which were in use by the task. In the course of normal operation that's handled by using the Mesos launchTask and sendStatusUpdate interfaces.