Pubsub service

[hairyhum]

Currently there are a forwarding service, which creates an fixed alias address per registered route and a stream service, which stores messages and can be queried to receive them.

Forwarding service is addressing the "discovery" issue, while the stream service is addressing the "reliable delivery" issue.

Another useful messaging pattern would be a pub-sub, when (possibly multiple) workers subscribe to one (or many) topics and receive messages sent to the topics without a need for reliability.

The prototype of such service was started in ockam topic

This work may be continued to integrate with Ockam Routing and protocol mapping.

Alternatively the pubsub service may be built on top of the forwarding service by allowing named subscriptions and multiple routes subscribing to the same topic.