[SPARK-4020] Do not rely on timeouts to remove failed block managers

If an executor fails without being scheduled to run any tasks, then `DAGScheduler` won't notify `BlockManagerMasterActor` that the associated block manager should be removed. Instead, the associated block manager will be expired only after a few rounds of heartbeat timeouts. In terms of removal treatment, there should really be no distinction between executors that have been scheduled tasks and those that have not.

The fix, then, is to add all known executors to `TaskSchedulerImpl`'s `activeExecutorIds` whether or not it has been scheduled a task. In fact, the existing comment above `activeExecutorIds` is
```
// Which executor IDs we have executors on
val activeExecutorIds = new HashSet[String]
```
not  "Which executors have been scheduled tasks thus far."