WIP: Dealing with intra gnmi-target run-time dependencies: data pot and pub/sub

In order to support a number of different operating system (OS) and application specific (AS) use cases, i.e., how to set or get information from the OS or an "applicaton", will require some changes to the target. From the yang side this will be handled by different yang models that are included in the Openconfig device notion, e.g., /system or /interfaces.

However, there will be dependencies between different (yang model) parts, e.g., an interface that is changing from up to down, will not only impact the interface itself, but also the routing part and potentially also AS code.

In oder to deal with dependencies in the configuration, the configuration should be stored in the device yang model as the central point (AKA the data pot) and interesting yang model implementations can subscribe to parts of the yang model watching out for changes. This will be done via a publish subscribe mechanis.