Bug fixes/enhancements from testing with MediaExport

Added the CONNECTION_STALL flag to the dlna flags set by HTTPServer
 in set_delivery_options_for_resource.

Added BACKGROUND_TRANSFER_MODE flag to the dlna flags set by MediaFileItem.

Have MediaFileItem set the resource size in get_primary_resource().

Fleshed out MediaResource.to_string() to aid with debugging.

Tested streaming and Range requests with Gst/SimpleMediaEngine.