split app-db and data-db maps

The fact that all dbs live in the same map makes it easy to pick whatever we want from that bag and we do the validation in cli-options.

Maybe though, we should think of having different maps, to have a clearer split. Or at least, "mount" them as "app-db" and "data-db" services.

The corner cases arise when we want the same app-db and data-db.

Start by reading the validation in cli-options and tracing how the shared database map is constructed and consumed. Map the app-db and data-db cases, especially when they refer to the same database. Done means the intended separation or service naming is resolved and the corner cases are covered.

Written by the indexing model from the issue text.