Key/value tables

[petehayes102]

As a publisher, I want to opt-in to retaining messages as key/value pairs, so that they can be queried by consumers.

If a consumer subscribes to the key/value topic, they would receive new and updated pairs only. If a consumer requests a single key from a topic (i.e. request/reply), the topic will send just that key, or None if it doesn't exist.