Performance improvements for user actions, fix scheduled tasks

[hopskipnfall]

At HEAD I've confirmed there is noticable lag at the following places:

• New user connects
• User quits a game

There is likely lag when various commands are run as well.

After removing all 90 calls to Thread.sleep(), this appears to be fixed. It's possible this causes bugs like:

• Messages being sent out of order (for their message number)
• Messages sent in quick succession (e.g. /help command or a user joins the server) end up out of order

[hopskipnfall]

This also adds the ability to specify messages to be displayed when a user joins or creates a game.