Resolve "The model stored in the plugin is increasing in size"

This commit makes sure that the stored model data is only unmarshalled
if the plugin is newly created. Otherwise we assume that the plugin
represents the intended state (this is only temporary and will be
changed in the future). Requesting configuration data directly from the
device will now only pass that information through. Only changes
triggered through the controller itself will be persisted.

Additionally the handling of a subscribe response has been changed so
the data is not unmarshalled into the plugins model data.