Improves startup and changes document deletion

   * Update proxy user in couchDB each time data-system started : It allows to be sure that proxy can access to database (for replication)
   * Stop data-system if couchDB isn't started
   * Stop data-system if it receives a unauthorized error
   * Use field _deleted (=true) to remove document from couchDB : It allows to send event *docType.delete* without use old revisions.