Support mutability/atomicity

[m0mus]

As an app developer, I want to read multiple configuration values which belong together in an atomic way.

Often business logic doesn't only need a single value but multiple values which belong to each other. like a server hostname, port and path.
It should be possible to read them in a single atomic access.
If we support mutability, we need to ensure the atomic access.

[m0mus]

Possibility to get multiple values from the config source in a single request.
We need a mechanism allowing a config source to notify that configuration is changed. Options: notification and direct call.
That notification mechanism must be switchable on and off.