v0.9.21

• graceful error handling of missing .bson file
• add more to preauth return: file size, options used
• added ability to download multiple files (.tar or .zip format) from a query
  • use 'download_url' option in query to get preauth url with archive of files
• add priority field to node along with index and set option
• add configurable max limit to revisions array
• enable preauth download (download_url) for subset nodes
• dockerfile update