Multiple workers are not supported due to locking

When running this under uwsgi with multiple processes (e.g. `-p 2`), you start seeing errors like this:

  File "./server.py", line 385, in save

    os.rename(list_path + "~", list_path)

FileNotFoundError: [Errno 2] No such file or directory: '/srv/serverlist/static/list.json~' -> '/srv/serverlist/static/list.json'

We do have a lock to prevent multiple threads from doing this, but not to prevent [multiple workers](https://uwsgi-docs.readthedocs.io/en/latest/Locks.html) from doing this.

Start in server.py around the save method and line 385, then read the linked uWSGI locking documentation alongside the existing thread lock. Reproduce the issue with multiple workers such as uWSGI -p 2; done means concurrent workers no longer produce the reported FileNotFoundError during saves.

Written by the indexing model from the issue text.