Enhance cms message writing to only output active messages

[jdalbey]

Currently the cms message json file contains an entry for every CMS, resulting in nearly 10K bytes being read every five seconds by every client. To improve performance we would like to rework the cms layer and the cms monitor so that the file needs only contain the active CMS messages. This will require a major rewrite of the logic for reading and writing messages.

Also, it would be nice if the logic were clever enough to recognize an empty or missing data file as meaning there are no active messages.

Also, perhaps #107 could be fixed at the same time.

[jdalbey]

I made an attempt to fix this and ran into technical difficulties.
One is that the json load is async which creates problems as show message doesn't wait and won't show the updated message.
Second is that when a message is cleared there's no way to detect it and reset the icon. Trying to resolve these issue got very messy so I gave up.
Now that we are using Apache on the production system, I don't think the performance is a problem any longer.