http_uploader: Add session ID.

- Sent as value for x-session-id header.
- Command line configurable.
- Defaults to current date/time.
- Update test_server.py to use session ID for output file name
  in non-DASH mode.

Change-Id: Ic69cc577d23cdb4902f927815a20c8378b9bdf18
4 files changed