[SPARK-4991][CORE] Worker should reconnect to Master when Master actor restart

This is a following JIRA of [SPARK-4989](https://issues.apache.org/jira/browse/SPARK-4991). when Master akka actor encounter an exception, the Master will restart (akka actor restart not JVM restart). And all old information are cleared on Master (including workers, applications, etc). However, the workers are not aware of this at all. The state of the cluster is that: the master is on, and all workers are also on, but master is not aware of the exists of workers, and will ignore all worker's heartbeat because all workers are not registered. So that the whole cluster is not available.

In this PR, master will tell worker the connection is disconnected, so that worker will register to master again.