Don't fail running jobs when decommissioned executors finally go away

When a decommissioned executor is finally lost, its death shouldn't fail running jobs. 

A decommissioned executor will eventually die, and in response to its heartbeat failure we will generate a `SlaveLost` message. This SlaveLost message should be treated specially for decommissioned executors: It should not be deemed that this loss is due to the running application. Decommissioning is an exogenous event and the running application shouldn't be penalized for it.