* Param verification on Feature -- it should be impossible to:
  ** modify a feature's param mappings so that they include two conflicting params
  ** install two features on a group if so doing would result in two conflicting parameters would be enabled in the group's configuration
* diagnose (qmf-related) failures related to events
* Implied subsystems on Feature
* Empty parameter to Node::CheckConfigVersion should produce an event for the
  node to check in for its configuration
* versioned configurations

someday/maybe:

- refactor transitive closure code (e.g. Feature#x_includes)