Cluster replication does not replicate users and grants.

[reltuk]

If we want a standby to be ready to promote to primary at any time, at least most of its user-facing configuration should be the same. This notably includes users and grants which the user will naturally assume replicate. Currently they do not.

We should replicate users and grants by default. If there are certain types of grants we do not want to replicate, we should come up with a use case and UX for it.