k8s feature: Leader election

[rvowles]

For people who run FeatureHub in k8s, we should have some way of indicating this so we can enable k8s specific capability.

Particularly there is leader election for the Management Repository. If we enable Leader Election, then ideally when features are updated, as part of the same transaction that will also insert a new record into a new "publish" table. The leader should monitor this table and publish data within it, making MR resilient to the situation where the messaging layer goes offline.

This would remove NATS from being a Health Check source and more of an indicator for MR to stop and restart publishing.