Rework sychronization of configurations

In the original approach configuration objects were not thread-safe. In later versions, more and more synchronization was added to support concurrent access to configuration properties, even when reloads occurred.

Unfortunately, the current implementation is not very clean and has a lot of problems. There a several bug reports related to this topic. Therefore, version 2.0 which will break backwards compatibility is a good opportunity to introduce a new concept for synchronization.

Of course, the new approach should be easier, cleaner, and customizable. I would also like to have an option to avoid synchronization completely if this is not required for a specific use case. Maybe a similar pattern as used by the Java collections framework can be applied: per default, configurations are not thread-safe, but there is an option to add synchronization so that they can be accessed concurrently.