DCP notification of durable persistence

This is a feature request to have Couchbase Server send notifications over DCP when document changes have been durably persisted.

One use case is for the Kafka and Elasticsearch connectors, which don't act on changes until they have been persisted to all replicas. The current implementation polls Couchbase Server using the observeseqnos command, but this is complicated and inefficient.

It would be great if instead of polling, clients could opt in to receive periodic notifications of the highest sequence number that has been durably persistence.