Support for agent lifecycle management.

This epic co-ordinates the work for introducing agent lifecycle management in Mesos allowing a framework to be notified in case of agent node failures. The existing Event::Failure is not enough for frameworks to know that the given agent node isn't ever coming back.

The primary motivations for introducing such a feature would be:

• Currently, when an agent running a task fails, there is inherently an operator interference needed (manual step) to remove the node via a configuration API exposed by the framework e.g., dcos cassandra node replace for the cassandra framework. This needs to be done once for every stateful framework running on the cluster.

• When an agent is marked as unhealthy, the removal rate is bounded if the `--agent_rate_removal_limit` option is set. This is specifically problematic for operators relying on EC2 autoscaling groups or for workload bursting to another cloud.

• When an agent is marked as unhealthy, the removal rate is bounded if the `--agent_rate_removal_limit` option is set. This is specifically problematic for operators relying on EC2 autoscaling groups or for workload bursting to another cloud.

• When the fault domain associated with an agent changes (e.g., it is moved from an unallocated rack to an allocated rack), there is no feedback mechanism for the framework.