Manager Configuration
---------------------
- configuration consists of one or more XML snippets
- each snippet is a <planet> node
- snippets can be loaded:
  - at start-up (on the command-line)
  - at runtime (through the AdminMedium interface)
- both call Vishnu.loadConfigurationXML
  - this calls:
    makeBouncer, addManagerPlugs, updateStateFromConf, and startComponents
- each snippet adds to previous snippets
- components with the same name in the same parent should "overwrite" the
  previous ones
- on each load, consistency should be checked:
  - before parsing, is the XML valid ?
  - after parsing, is the cumulative configuration valid ?
    - do the feedId's referenced by <source> tags exist ?
      (FIXME: right now, updateStateFromConf calls mapEatersToFeeders, which
       checks for components referenced by source tags that do not exist)
- errors at any stage should also be viewable from admin clients:
  - component-specific config errors can be added as messages to the components
  - manager-global config errors can be added as messages to the toplevel
    planet object

