Add cleanup function in `message-queue.js`

[wkande]

When the Nodejs instance (running with PM2) shuts down, call finalizeDeleteSystemMessages().

There are timeouts running that remove system messages such as the /chatrules. These message need to be removed from the group before shutdown. The msgIds to remove can be held in a simple JSON object.

{
   <msgId>: <timeoutValue>,
   ...
}