Collections LIST command should fetch ZK data, not cached state

The LIST command to the Collections API presently returns all collections from the cached ClusterState. The proposal here is to fetch the child nodes in ZooKeeper instead. While the distinction is minor, the intent is to improve robustness / comprehensiveness. It may mean a collection is listed that is being created or being deleted that otherwise might not have been returned. The scenario driving this is to facilitate cleanup daemons that identify potentially left-over data from a collection creation or deletion that maybe failed (partial create or delete).