MultiServer: Lock support

[Berserker66]

What feature would you like to see?

Adds a new client -> server command: !lock

If used without arguments, collects all uuid's of the currently connected clients to this slot and saves them as locks on the server, the server then responds to the clients of this slot that the slot has been locked with the following passwords/identifiers and lists them. We may want to ignore uuids that evaluate to False, to filter out None, empty string, 0 etc.

If a client connects to a slot initially, if there is a lock, then either its password field or uuid field needs to match one of the lock values to be Connected

If the command !lock is used with a text following it, then that is saved as identifier instead, to be used as an explicit password.

If the command !lock false is used, then the locks are removed.

Server side, /lock is added, which functions the same but has slot_name as the first argument to select which slot to affect.

Optional:
It may be a good idea to lock the locking feature behind a server option, to prevent people locking slots in an open game.

[duckboycool]

I don't know that this would be all that likely to be an issue, but I think it'd be better for a separate command (!unlock?) instead of !lock false since this could be confusing or done accidentally considering that the argument would otherwise be a text password.