HeartbeatReceiver doesn't remove lost executors from CoarseGrainedSchedulerBackend

When a heartbeat timeout happens in HeartbeatReceiver, it doesn't remove lost executors from CoarseGrainedSchedulerBackend. When a connection of an executor is not gracefully shut down, CoarseGrainedSchedulerBackend may not receive a disconnect event. In this case, CoarseGrainedSchedulerBackend still thinks a lost executor is still alive. CoarseGrainedSchedulerBackend may ask TaskScheduler to run tasks on this lost executor. This task will never finish and the job will hang forever.