Improve Platform-Importer

Nuxeo Importer is a mess :

• the initial code comes from the first RMI based Shell (Nuxeo 5.0)
• since then we have done a lot of evolutions
  • scan-importer
  • complex-metadata mappers

This basically works, but :

• the framework is more complicated that actually needed
  • Service, Component, Singleton : this needs refactoring
  • we should provide a single bundle with various configurations
• some configuration don't work as expected naively
  • import of a big folder is complex to do correctly
• configuration should be possible without doing java code
• allow to manage several importers at the same time