Churn metric

[coot]

We need to design a churn metric to retain the most valuable connections.

Proposition

We could start with a metric which is similar to upstreamyness metrics: count the number of messages first advertised by a peer, which were later proved to be valid (invalid messages must be ignored).

This will require changing the tx-submission to a protocol with reversed agencies like the object-diffusion protocol for Peras. Note that the object-diffusion protocol must be updated to enable client-side timely demotion.

Since this requires a protocol change, the new protocol must be negotiated in a new node-to-node version; connections that negotiated tx-submission can be at the bottom of the metric.

NOTE: this change requires updating the CIP#137.