Provide a solution for SNS requirements

The current configuration, and certainly content, require SNS support.

Although some use-cases, like for hipposys:authrole (HCM-26) can/may be solved without SNS support, there are many other use-cases where we likely will need to support SNS anyhow. And for document variants (in content, but also for (query) prototypes) we definitely need to support this.

When we add SNS support, to be done through a follow up issue, we (then) need to be able to detect and distinguish between merging definitions (the current default) from appending definitions (resulting in SNS).