Rewrite the archiver

[daniel-j-born]

• Upgrade to Python 3.
• Reuse a requests.Session() object to reuse connections to increase archiving speed and avoid being spammed by Yahoo.
• Pause between requests and exponentially backoff on errors to avoid being spammed by Yahoo.
• Change user-agent to avoid being spammed by Yahoo.
• Write messages to groupName/year/month/msgid.json instead of groupName/msgid.json.
• Write to a tmp file and then rename into place to ensure no data corruption.
• Create output directory in current directory rather than source code directory.
• Change logging to Python logging.
• move_to_year_month_dirs.py: New script to rename groupName/msgid.json to groupName/year/month/msgid.json.