Clean up NetconfDeviceTopologyAdapter datastore lifecycle

NetconfDeviceTopologyAdapter is using 'mergeParentStructure' operations to ensure the underlying topology is already present in the datastore. This is wasteful, as this is an invariant that the callers should be taking care of before instantiating the adapter.

Furthermore, the adapter performs writes and merges which seem to be overlapping, perhaps introducing state without regard of what has been done previously. We should optimize this by changing only what needs to be changed w.r.t. baseline.