Enable deletion of namespaces

[andrewjstone]

Deletion requires shutting down all the replicas. We need to ensure this is done safely so that they don't come back to life.

Option 1) Gossip and a delete flag -
Problem: What happens if a namespace of the same name is created concurrently with the namespace deletion process.

Option 2) Two-phase commit for atomic deletion.
Problem) Expensive. All nodes must be online and reachable.

Option 3) Root Consensus group for all create and delete operations.