Refactore features support for more reliable sync behavior

The distributed Map used to store the features on the cluster is not manipulated consistently by the different Cellar modules.
Instead of using Map<FeatureInfo, Boolean>, I'm now using Map<String, FeatureState>, where the String key in name/version and FeatureState contains name, version, and current status (installed or uninstalled).
It allows a largely more reliable behavior.