Handle a network partition between Master and Slave

If a network partition occurs between a Master and Slave, the Master will remove the Slave (as it fails health check) and mark the tasks being run there as LOST. However, the Slave is not aware that it has been removed so the tasks will continue to run.

(To clarify a little bit: neither the master nor the slave receives 'exited' event, indicating that the connection between the master and slave is not closed).

There are at least two possible approaches to solving this issue:

1. Introduce a health check from Slave to Master so they have a consistent view of a network partition. We may still see this issue should a one-way connection error occur.

2. Be less aggressive about marking tasks and Slaves as lost. Wait until the Slave reappears and reconcile then. We'd still need to mark Slaves and tasks as potentially lost (zombie state) but maybe the Scheduler can make a more intelligent decision.