Rculbertson/tombstone rebase

With this PR, we no longer use tombstones. Instead when a job is removed, the master will delete the job from zookeeper. To protect against the case where operational error causes zookeeper to lose all data, or we aim an agent at the wrong zookeeper cluster, we check for the existence of a cluster ID node. If this node is not present, the zookeeper client will throw an exception, and the agent will not sync down the latest jobs.