Removed dead instance cleanup from InstanceManager so that Akka's watch mechanism is the current mean to detect dead instances.
Made ExecutionGraph, Execution, ExecutionJobVertex, ExecutionVertex, AllocatedSlot, Instance, CoLocationConstraint, SharedSlot and SlotSharingGroupAssignment serializable. Integrated Kryo to be used to serialize Akka messages.
Fixed AccumulatorEvent serialization problem. Registered additional classes with Kryo.
Add debug guardians