Update the agent's behavior when marked GONE

Currently, when an agent is marked GONE, the master sends a ShutdownMessage to the agent, which causes it to shutdown all frameworks and then terminate.

As part of the agent draining work, we would like to change this behavior so that instead of terminating, the agent will sleep indefinitely once all frameworks are shut down. This will avoid the issue of a flapping agent process when the agent is managed by an init service like systemd.