XDCR - potential issues with target-side quorum failover

ns_server introduces a feature called "quorum failover" in CC (7.0)

If a cluster is a target cluster of a XDCR replication, and a quorum failover happens, there is a small chance that issues may arise.

MB-45435 was the original ticket that tracked the XDCR issue and the document link in MB-45435 contains some technical points that describes the issues that may arise when a target cluster performs quorum failover. The possibilities of these happening is quite small, but yet not impossible.

Some of these issues could be quite severe, as data may be replicated to the incorrectly mapped collection, which may violate certain production environment. As an RBAC example, a userA's collection's document may be replicated to a userB's collection, which is an access violation.

The goal of this doc ticket is to try to clarify the intricacies involved not just with quorum failover, but quorum failover as a target cluster, such that customers at least will know the issues that may arise and Couchbase will be somewhat protected when abnormal behaviors such as these show up.

Since the concept involved is a bit convoluted, a release note may not be enough.