Detect hard crashes of workers using a heartbeat mechanism.

Also fixes some issues in the rest of the code with detecting workers this way.

Conflicts:
	core/src/main/scala/spark/deploy/master/Master.scala
	core/src/main/scala/spark/deploy/worker/Worker.scala
	core/src/main/scala/spark/scheduler/cluster/SparkDeploySchedulerBackend.scala
	core/src/main/scala/spark/scheduler/cluster/StandaloneClusterMessage.scala
	core/src/main/scala/spark/scheduler/cluster/StandaloneSchedulerBackend.scala
Some bug and formatting fixes to FT

Conflicts:
	core/src/main/scala/spark/scheduler/cluster/SparkDeploySchedulerBackend.scala
	core/src/main/scala/spark/scheduler/cluster/StandaloneSchedulerBackend.scala