feat: Pub/Sub link-aware publishers

[merklefruit]

In order to implement link-aware optimisations (for instance, decide to compress messages over a link only if the latency over that link is greater than a specific threshold) we could implement the following:

• all Publishers also subscribe to a "control topic"
• all Subscribers also publish to that topic

Since every sub socket already retains its Stats, we would only need to publish aggregated statistics on a regular interval or every X messages received (aka "sampling").

This feature will allow for smarter, link-aware publishers.