Scope pubsub topics to the cluster namespace.

[lthibault]

Topics are currently scoped globally, meaning that a name collision occurs between the topics joined by the PubSub capability and the cluster namespace. For example, calling n.PubSub().Join("foo") against a node started via ww start -ns foo will return pubsub.capnp:Topic.publish: topic already exists.

To remedy this, we should add the cluster namespace to each topic as a dot-separated prefix. Thus, Join("foo") from our previous example would produce the fully-qualified topic foo.foo.

Interestingly, we might use this mechanism to support an empty "" that is nonetheless distinct from the cluster topic (it would map to the topic .).

[lthibault]

Implemented in wetware/rs@fe32261.